What is it with anime and this trope

Fair enough. I won't argue that. Some are really really bad with that.

There are ways to make this slightly less black and white, but most of the time the author doesn't do it because giving the main character mental health issues / memory loss takes a considerable amount of effort and can easily come off as making fun of the irl equivalents.

If the main character only had the memories of their previous life, none of the personality, extremely little of the wisdom, none of the muscle memory/ automatic responses, and pretty much needed flashbacks to remember anything from their previous life (eg. remembering witnessing someone's death being a way of remembering that walking alone at night is dangerous, potentially also having modern police or weapons be shown leading to the main character questioning where these memories were from) or at least extreme emotion attached to the memory (eg. The main character encountering fireworks, making them feel happy/ nostalgic while their peers are terrified of the loud noises). Essentially what I guess I'm trying to say is in my opinion if the previous life was treated like an illness instead of just who they are cross age relationships in isekai would have more complexity to their morality.

FMA: Brotherhood was good af!! As for my actual favorite..... I'd say either Baccano or Welcome to the NHK.

Edit: adding cowboy bebop as tied for first as well. Always a classic.

Because I only watched the mainstream ones and FMAB is probably the best of them.