Actors playing roles on TV series occasionally leave the series after a while, for various reasons.
I've noticed that, at least in the last few decades, that usually results in the character leaving the series as well (by dying, leaving the city or in different ways). While this makes sense to me for minor characters, it seems to be the same way for main ones. Even if the character was the Chosen One and haven't fulfilled their destiny yet, it seems the creators prefer to find a way to "bend the rules" and have a new Chosen One than cast a new actor to play the same character.
To me, this makes little sense. Even if they can't find a way to explain why the character looks different (such as in Doctor Who) I'd expect it would be easier in most ways to continue filming with a new actor than having to change future scripts to allow an excuse and introduce a new important character. I'd also expect that viewers would easily understand that different actors can portray the same character.
Is there a legal reason for the above? If not, why do creators prefer to go for a more complex solution than a simpler one?
Note: I am aware that the above is not always the case, but it does seem to be more prevalent.