This trope is often considered "overdone" today and fought kgthought of as uncreative... but it has a legitimate place given viewer tendencies.
Media is often ignored if the very beginning isn't interesting. Think of how often a song is skipped if the first two seconds aren't immediately captivating.
Consumers today desire constant stimulation... and so many stories will start "in media res," or in the middle of it.
Producers know that first impressions matter. Thus, the heart of the movie is shown very first to compensate for the fact that modern viewers judge from the first second.
Not many want to wait to find out the plot or general direction of a film. That's why franchises are often repetitive or based on well known series from other forms of media. People want to be familiar. So bridging some familiarity in the beginning helps audiences.
There are other reasons, but that's the one I wanted to address.