Unless the system doesn't have enough memory to allocate for large compilation units and starts swapping allocated/dead memory and slowing down massively. Unless the user runs a CI where as many things should be compiling at the same time as possible and slowly freeing memory in 3 concurrent processes is better than running only 1 at a time. Unless they have spare cores, but no spare memory. Unless ...

> They just care ...

It all depends on who "they" are and what they want to achieve.