Cross-gen is a system where old generation games can be used for new generation consoles. This is happened especially to make fans can adapt to the new system. Moreover, certain games are usually more fun to play because the console performance has increased.
Reporting from Gamerant, Sony gave responses At least a cross-gen game from PS4 to PS5. This was revealed by Eric Lempel during an interview with Geoff Keighley, during the release of the DualSense Conroller for PS5. Eric Lempel here explains as Head of PlayStation Worldwide Marketing.
Eric gave a response that the game in the latest generation was created specifically only for PS5 technology. Similar to the case of the Dualsense system which is applied to PS5 games, of course, it cannot be applied to PS4.
Especially if there is a PS4 game on PS5, the technology might be very different, one example is GTA 5. Where the GTA 5 PS4 version will be very different from the GTA 5 PS5 version because of differences in console technology.
Therefore, the explanation above provides a pretty good illustration of why there is a lack of cross-gen between PS4 to PS5. Although its own rival Xbox Series X actually does the opposite by multiplying Cross-gen and instead prohibits developers from asking for additional fees for Cross-gen.