The developers can, or they can add a toggle. It isn't fundamental to UE5. ARC Raiders and Squad are both on UE5 and don't use Lumen.

The issue is supporting Lumen and another lighting solution requires them to make sure both work. For multiplayer games especially, having both isn't an option, because then it gives an advantage to some people. Squad, for example, looked into it, but they ended up going with a different GI system that's more performant so everyone can (and must) use it.

For single-player games, it's possible to have Lumen and another option. It's just extra cost to development. They'd rather go with the option that creates better trailers and not worry about people struggling to run it. They can run at an upscale 240p for all the executives care.