Why the fuck not?? Has there been any explanations for any of the games so far?
The closest thing we know is that PS4 and PS5 games are treated as different SKUs and Sony has a very restricitve policy when it comes to cross-saves between different SKUs. I dont remember the details sadly, was discussed in another thread.
The last theory I heard is that PS4 games that run as PS4 games via BC should be able to use old saves but if games get patched to become a true PS5 Version it gets extremely muddy.
Just a theory though so take with a big grain of salt.