Why are there so many Fighters and Mages?

Because "Fighter" and "Mage" are both incredibly generic terms that include too many champs.

seems to be easier to make more diverse fighters and mages than the other classes.

i mean look at the difference between garen, and yi who are both fighters but play and build entirely different.

and then look at the marksmen for comparison, almost all of them buying the same items and based around auto attacking. there seems to be much more overlap compared to other classes.

riot only recently managed to make them a bit more diverse at least concerning their kits.

Because time has shown that these are the champions people like playing most. As was mentioned above, they're also very arbitrary, with 'Fighter' simply meaning 'Any melee who is not a tank.'

Roles are a bit obsolotes Jax can be played as an assassin, even if he's very resistant etc etc

Because fighter and mage are too generic. If you separate Juggernauts from fighters, it becomes are lot more reasonable.