This IS what character classes represent and facilitate. Having a character class means they have trained just enough to become competent in that vocation—the assumed amount of time it took would be different between classes, but the wizard can cast a spell, and the fighter is not wholly defenseless with a weapon. This is (or can be) interpreted to explain why classes are barred from certain skills or abilities; game mechanics aside (the real reason), would a wizard who spent a decade or more to learn offensive spells, take [ie. waste] the time training to become proficient swinging a sharpened stick?—and be okay with being seen to win not by use of magic, but by hitting their enemy with a rock? Vise versa with a veteran warrior being reduced to using cantrips to win duels by —cheating—.
It extrapolates not why they can't learn to use it, but why they would never choose to learn to use it.
Limited respec? The simplest way is to not give the option, and have the player accept people as they are. Using Minsc as example, suppose that odd level intelligence was to —ensure— they could not gain the bonus immediately.
The worse problem is the developers who design respeccing into the game from the outset, even building the gameplay around it! It limits and cheapens the PC, and NPC alike, for no commitments have any weight —or teeth when they are decided. Nothing is written in stone, and anything can be backpedaled out of.
That is when the characters truly become just a list of optional numbers; interchangeable ones at that. For with that, one can no longer write a personality based on the statistics; the game doesn't hold you to their consequences.