It's not a super simple task, it's not as seamless as documentation makes it appear. While there is backwards compatibility, there is also isn't alot of backwards compatibility. Still have build certain systems and code base with UE5 in mind, as a lot of things are UE4 specific and very different in UE5, and then adapt that to the new engine codebase when you switch over.
A lot of differences in meshes, world building, lighting, water tech etc are also present in UE5, which requires manual work to adjust from UE4 standards to UE5 standards.
UE5 is no silver bullet, it's not going to fix every major issue with the game, but will improve workflow and offer a lot of stepping stones for future improvements, when UE5 matures over the coming years.
I can see how it would be something more than just pressing a button to migrate UE4 to UE5, it would be naive to think it would be something fully automated.
But the real questions are, what will UE5 do for mortal in a short term to make it better. Will those development resources compensate for the lack of current design and content? Is the timing correct?
I've seen Henrique say in a stream that the actual migration from UE4 to 5 won't "drain" their development resources. Now i find that hard to believe since when they are supposed to be fully focused on developing something it comes out in the form it does, placeholder poop. Imagine if they actually have to multitask instead of focusing on one thing.
Idk maybe im missing something here, but Henrique was talking about how good" would water look with UE5, who gives 2 fucks about ue5 if the game is garbage and they can't manage to deliver good mechanics.
Or maybe i don't understand UE5's truly potential and how will SV capitalize it in short term, but as far as i see this is just a bad excuse for not fixing the placeholder garbo content its in the game.