I've expanded this, because I don't think the accepted answer actually answers the question at all, or if it does, it's from the wrong perspective; that of the audience, not of the industry.
A movie is a pyramid.
There's only one keystone right at the top and a whole slew of other equally important stones beneath, holding it up.
 As pointed out in comments, arches have keystones, pyramids have capstones.
There's also the consideration that 2nd unit director is not, in and of itself, part of a career path. It may be a stepping stone, or it may be a chance to direct without the totality of overall responsibility for those who otherwise wouldn't - e.g. Frank Marshall, a huge Hollywood producer who just happens to be best mates [& co-CEO of Amblin] with Spielberg. It may be that the 2nd unit is directed by someone with a good insider knowledge of the overall look and feel of the movie - Andy Serkiss for example on the Hobbit movies. A good few 2nd unit directors are fight co-ordinators. Put them in charge in an area where they shine.
This line from the wiki really bothers me…
Some who became directors may return to working predominantly as second unit directors for the remainder of their career.
This implies that unless you become a world famous director in your own right, you have somehow failed. That actually makes me angry. There are a thousand important roles in a movie, even if usually only the director & first few names on the cast list are generally known to the public. Without those others, the movie would never get made in the first place. It's the public perception that fame is most important criterion that skews this sentence.
"Ooh, that Roger Deakins, only a glorified cameraman, never made it beyond DoP, what a failure!" is never said by anyone, ever, anywhere.
As a career path, you can't just kick and scream your way to the top, you can't really climb over others to do it, people you offend on the way up will always be there to greet you on your way back down.
Not every other level of the pyramid wants to end up at the top. Many will stay at one level their entire careers. Think about the myriad set builders, painters, carpenters, the lighting crew, locations, services. Or the 'lesser' artists, supporting actors [extras], day players, 'bit parts'. None of those will ever go on to direct.
There are recognised paths to becoming a director; some people bounce out of film school thinking that they're about to be the next Hitchcock, in less than 5 years. They're usually ground down in less time than that. Real life gets in the way once you've done the easy bits in college.
I have met a fair number of people who "write and direct their own movies"… it's a hobby. They've been doing it 20 years & haven't sold one yet. It's either just for fun, & why not, or it's delusional. So far, that's a 50/50 split. Sometimes I'll help out on the 'fun', usually for free. I won't go near the 'delusional'.
It can start as a 3rd AD, whose job it is to keep the director happy. They direct and track the movements of any supporting artists, modify their routes through frame as the director wishes, and try not to get either themselves or the artists they're in charge of, in the way of the director's vision for the shot.
Most 3rds leave the industry before going any further. A 30-year old 3rd is 'old'. A very few go onwards and upwards. The traditional next step is to become a 2nd AD. The trouble with that is that the 2nd AD is more of a producer than a director and may never even see the film set at all, spending their time back in the production offices making sure things happen on time and people get paid. This is not a job for everybody on their way up, though for some, it's a step towards production, for others it's an annoying couple of years away from set while they're trying to get the break as 1st AD.
Some of the clever ones manage to skip this step. Often this is because they've been pushy enough to have been given the second unit to direct on occasions. A foot in the door. This would be a tiny unit; possibly only to film establishing shots, though occasionally some small dialog scenes. This would not be the 'Hollywood' big independent second unit doing all the fight scenes. Much smaller.
Having got this bit of the career path under their belt, it's then much easier to become a 1st AD. The 1st is the person in charge of everything on the film set, under instruction of only the director above. Their job is to co-ordinate all the other departments to enable the setup to be ready to roll. They are then responsible for the actual calling of the roll as filming starts.
The 1st AD is known as "first below the line". The 'line' is what distinguishes technical from artistic roles. Director, camera operator, DoP [director of photography], all artistic. Lighting, sets etc., technical.
Once established as a 1st, even at that height, most people then stay in that role. A few move towards production, a few towards directorship. Second unit then becomes part of that stepping stone again.
The other path towards director is via camera. AC3, AC2, AC1, camera op, DoP… 2nd unit director, director. That works above the line most of the way through.
So, whilst the big famous name is right at the top of this pyramid, all the rest of it, completely unnoticed by the audience, has their own hierarchy to support it. Some working their way up, some staying where they are.
A quick side-step into TV production - these days under the guidance of 'show runners', different units will be used in 'blocks' each with a different director, in charge of a couple of episodes. Sometimes this is otherwise the same crew, sometimes an entirely different unit, working in tandem.
For example, The Crown is shot with two entirely different units - known as King and Queen - working simultaneously throughout. Two sets of directors working to achieve a single vision. Neither of them is in charge of the overall look and feel, so you could theoretically consider them both second units.
In the five years that's been running, one guy who started as Crowd 2nd [making sure the 'extras' are all on set at the right time] became 'full' 2nd AD, then has started filling in as 1st if either of the others are absent. One chap who started as 1st in season 1 is now Producer, though occasionally still works as 1st.
Onwards and ever upwards, for some.
I also know two 3rds who are now 1sts, but so many of them over the years just left the industry altogether. It's a tough climb.