It’s not that they can’t, but they just don’t have the budget to.
UE5 is meant to be tailored for each game. But if you are using an off-the-shelf engine, that likely means you don’t have the ability or funding to do so in the first place. And those that do have the budget and means to tailor UE5 would rather just uses their own engine instead.
Hence we end up with a lot of UE5 games that runs really badly.