Originally posted by magnificent moose:

You can optimize LODs, lightmaps, asset streaming etc so much better when the exact content in the game world is known in advance. Procedurally generated games such as Valheim do not have this luxury.

You can still optimize LODs, lightmaps and etc. The problem is, that, for example.. In a static world game, you, as a dev, can make sure that the most resource-expensive features are never spammed tightly in the same room, near each other.
In dynamic world games like Valheim, users can do that on their own, breaking any levels of preparations developers could have. For example, lighting is a very power-consuming thing, so most static world games pre-bake the light into the world. You really cannot have such kind of luxury a dynamic world games. That makes all your lighting dynamic - the most power-hungry type of lighting. Basically, an RTX raytracing level of power-hungry.
If you add more than 3-5 sources of dynamic lighting in the same frame of the screen, you'll get a very poor performance even on the beefiest GPUs there are.

Then, you apply physics calculations, which is an another power-hungry feature not many games are brave enough to adapt.

The world being dynamic adds its own layer of recalculations, so it is also power-hungry.