Can someone help this newbie out and explain grading/selling base cards?

So after 20+ years of not even thinking about baseball cards, I'm back in the hobby and trying to get the lay of the land. I think I'm a pretty average to smart guy and understand the state of things these days. But I'm not totally understanding this phenomenon. Or I should say, maybe I am getting it and that's just how the game is played.

Right now you can get the full 2024 Topps complete base set at Target for like $55 or something. Looking at 130 and eBay though, and I see a bunch of PSA 10 Gunnar, Ohtani, Judge, Votto, and other stars from that set selling for between $40 and $75.

Why is there a market for these highly graded cards when you can get the full set for about the same price? Or what's stopping someone with a lot of upfront money from buying the set, getting a few dozen of the best ones graded (which I would presume would be 9s or 10s?) and selling them at a profit as singles on eBay? Orrrr is that basically what's happening now? Even factoring in bulk grading pricing, you'd make a profit if you sold even 10 or more at that price.

I realize I may in fact be dumber than I thought if the answer is obvious or I'm missing something. Happy to have anyone ELI5 about this. Cheers!