I mean this idea keeps resurfacing over and over. It seems a sizable portion players just plain want to fold all classes into just fighter/cleric/wizard/rogue. I don't really get the appeal or even the potential advantages of this.
Have you ever played one of those games that goes into super granular detail for describing damage, where it actually matters what type of damage is dealt to which part of the body, because if your arm loses half of its HP to slashing damage then you're at -3 to parry rolls but bashing damage to your head gives a penalty to concentration checks?
It's not that anyone really thinks it's a bad
idea to differentiate between damage types and body parts, but the cost associated with implementing those rules is prohibitive. Combat slows way down, and you have to do a lot more bookkeeping to track penalties, and most people think that it's just not worth it. They'd rather just write down the damage, describe it however seems appropriate at the time, and get on with the game.
Classes are similar. It's not that it's necessarily a bad
idea to differentiate between magic-users who learn from a book and magic-users who cast intuitively, but it adds a lot more complexity to the character creation process for very little benefit. When you add in multi-classing and feats, you get a combinatorial explosion that can take months to figure out and a week-long process to build a character
after you understand how everything works. I played Pathfinder for three years, and I have no intent on going back. It's not like there are any character concepts which
require mechanical differentiation to represent; if I want to describe my magic-user as casting intuitively rather than through study, or even through music, then that's a detail I can work out with the DM. (Additionally, from a setting-design standpoint, we don't necessarily
want a world to have both wizards and sorcerers in it; in much the same way that we don't necessarily
want mutants in the MCU, since we already have Inhumans who can fill the same role.)
So the short answer is that the cost of implementing these options outweighs the benefit, from the perspective of the person making the argument. It appears to be a fairly common opinion, particularly from those of us who survived 3E and 4E and now want to move on. We don't want the game to be
about that one specific thing, so it's preferable to gloss over it so we can move on to the interesting things.