There have been some serious bumps on the road to its release, but the PS5 hype is still high. However, even die-hard fans of Sony's systems aren't happy that some PS4 saves won't transfer with their PS5 upgrades.
Players generally try to focus on a single system, and when getting a PS5, players want as many upgrades as possible over PS4. The smoother the transfer from PS4 to PS5, the faster the older system can be passed up completely for the newer. A big part of that is transferring games from one system to the other, so only one system is necessary. A lot of people have wondered why the PS5 doesn't have more cross saves for its titles. It's especially important during a console's early years, when it has few big, must-have games to carry it.
In a recent blog post, Sony directly addressed the question of transferring saves from the PS4 to the PS5. Sony stated that saves could be transferred through several methods, including LAN cables, WiFi connectivity, or even a USB flash drive. Of course, PS Plus users can sync saves directly through cloud storage. However, on the matter of which games players could transfer saves from, Sony had previously been unclear. Some games, like Spider-Man, will not transfer saves from PS4 to PS5. Sony stated it's up to the developer whether saves are transferrable or not.
More specifically, Sony stated that while the save-trasnfer capability is there, it is up to developers to take advantage of it. The save-transfer function is not mandatory for developers, so players will just have to hope they use it for any games going to PS5. In some cases the answer seems to be yes and no, such as Spider-Man not getting a cross-gen save, but Spider-Man: Miles Morales definitely getting one. This makes sense, since Miles Morales is touted as one of the games to own on PS5.
In other cases, the answer is definitively no. For example, Dirt 5 will not have cross-gen saves, and that is not likely to change. Many developers likely feel it is simply not worth the effort to make saves carry over to a new system. Players will probably not throw out old PS4s right away, if at all, so games can still be played on it.
Of course, the other reason is a bit more businesslike. Sony would no doubt prefer that new games sell on the PS5, and a good way to do that is limit the old games players can play on the console. Without saves from old games, players will have more incentive to buy new games for the system.
Source: Sony