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.

High 50s isn't actually an issue for any modern CPU I know of

Yeah, that's almost certainly not because of Unity. At most, it could be blamed on C#, if we're blaming the technology. This is an issue with their simulation I would assume. For example, Timberborn is simulating liquids and a population of workers. The liquids are probably the biggest culprit, and there's a reason you don't see many games doing it.

All the games you listed are simulation games though. They are going to be the largest CPU hogs you can get, especially when you use the highest simulation speed possible. At that point, they're usually literally maxing out your CPU and running it as fast as it can process. As another example of this, Paradox games can not reach their highest speeds on weaker systems or later into the games. They run as fast as the CPU can process, which means nearly 100% utilization. It's not because they aren't efficient. It's because you're telling it to go all out on processing.