I am going to say bull hockey on the wizard being like a grad student who studies all the time that may be true about things magical but what about the other knowledge skills. Whose to say that the sorcerer or the monk or even a fighter does not spend their downtime reading everything they can about history or dungeons or religion. I know people in real life who have never really had former education beyond high school who are self educated just because they like learning. One of my family members is a deputy sheriff he has a two year degree in law enforcement his hobby is the civil war he does reenactments. He has read hundreds of books on that time in American history he has actually been invited in to give talks at the local university on the subject of the civil war.
I have seen people choose not to play their wizard with a lot of knowledge skills which is fine like I said PCs should not be cookie cutters that is so boring. And even if the sorcerer chooses to be a history buff they are stepping on the wizard toes since skills and wizard magic come from intelligence they usually have a bigger intelligent stat than the sorcerer so that means extra skill points and bigger ability bonus.
Like I said it bothers me that skills are based on class abilities it is like these PCs never had a life before they became adventurers.
Its not that PCs cannot learn skills that are not part of their class- or, people IRL, their job or profession- its that those classes have as part of their training some formal education in those skills.
My mother is a music teacher, and she knows a lot about music history. I'm not a music teacher, and I know nearly as much as she does, and am actually more knowledgable than she is within certain genres. She knows a lot about music theory, and I know virtually none. But I play 3 instruments and sing; she does voice & piano only.
Similarly, my mom and I both design jewelry as a hobby, but she knows more about grading pearls than some of the jewelers in the area...and as a result, they ask her opinion in pearls with regularity. Me? I know jack all about pearls.
Bringing this back to classes & skills... Classes do not prohibit you from taking any skill. They just describe the skillset that the class is most likely to provide, and therefore, gives you an advantage in learning those skills.
The average wizard, by the nature of the training he requires to become what he is, is an academic researcher. The class gives an advantage to, but does not demand, Wizard PCs learning knowledge skills.
This does not mean that Joe Sorcerer cannot learn the same kinds of things, just that he has a different, non-academic focus.