If i play a 5 years old game on a new GPU, i can blast it in 4K, no reason why they can't push for that on consoles with (minimal) work, like Xbox one X BC does anyway, you can play Xbox 360 or even OG Xbox games in 4K.
Why would Sony do this on some of their top shelf products for free? Because it's good publicity, it reinforces those brands, it reinforces their brand in general, and it pushes new people to try out those games in the first place.
Sony is putting 90% of their eggs in their "exclusives" basket, so nurturing those exclusives through time, is entirely within that strategy.
Really, pushing BB to run in 4K on PS5 with a patch can't take all that much work.