Some developers like Konami are greedy and remove the compatibility for certain games so that they don't work on the PS5 and the only option you have is to buy the PS5 version again.
The thing is, the government of California had overwhelming evidence that would've seriously dented Activision's reputation and cost them way more so they could've gotten a $500m settle deal. Instead, they settled for a paltry $18m...
At the moment, it's best to get a pre-built system as the price for computer parts is completely bonkers. Or you can look into a gaming laptop but build a PC at the moment is very expensive especially for the average gamer.