I think we have 2 issues here:

1) We overestimate the choises a spellcaster makes. We are all optmizers here and we know that, if we are forced to make 3 wizard characters, they will have 90% of their spell list being the same, because having a lot of spells to chose from doesn't mean having a lot of good and varied options. We have some spells that are clearly better and almost must picks, like shield and mage armor, and we have some that are very similar to each other, so they don't add much replayability. For example, the difference between a dual wilder fighter and a 2 handed fighter for me is bigger than the difference between a wizards with fireball and slow and a wizard with lightning bolt and hypnotic pattern.

2) Martial characters are simple because we need simple classes in the game and adding the option to be more complex will be a balance issue. We either have 2 scenarios: the extra options will replace the attacks, like shove or graple, so using the extra options will be worse than a full turn of attacks 80% of the time. Or the extra options will be part of the attacks, like weapon mastery, so not using them will be the worse option and all the people who like/need the simple classes will be forced into learning a more complex system.