It's just poorly coded and doesn't take advantage of the CPU power on hand. A lot of games are like that, both indie and AAA.
Poorly coded? RimWorld handles hundreds of mods mostly gracefully. Combine it with rimpy GPU compiling textures and you shouldn't have fps issues unless your computer is a potato or you have tons of VRAM heavy mods.
Yeah it’s notoriously cpu bottlenecked and doesn’t do much multithreading