Whether or not a spellcaster can be "simple" oftentimes comes down to just how many rules the game gives to casters the player actually engages with.
You can make a Wizard right now that uses merely a couple cantrips and a single 1st level spell that can be more powerful using higher-level slots. Sure the game allows the player to select a whole bunch more and allows the player to change out what spells they have "prepared" each and every morning... but no one HAS to do that. A "simple" Wizard just has Magic Missile prepared and the player casts nothing but Magic Missile over and over and over again, using whatever spell slots they have.
Why is that "bad"? In the minds of many people,. I suspect it's because too many of us players have this personal bugaboo wherein anytime we feel like we have to personally nerf ourselves with the rules to give us the experience we want, then we are "bad players". We aren't pulling our weight if we don't use the rules to their fullest potential. It's an ego thing. We don't want to feel like we aren't playing well by not using the tools the game provides us.
So rather than restrict things personally to create the experience we want... instead we always ask for WotC to do it for us. If THEY produce a "simple spellcaster", then we can use it happily and play that simple caster to the best of our ability under the restrictions WotC has put it under. And now you can't blame us with how the class plays... because this is what WotC has provided! It's not our fault! It's just what the class does!
For some people that excuse is preferable than having to tell their fellow players they are restricting themselves of what they can do, all because there's a gameplay loop they wish to experience.