So I was installing cargo-edit, and when it was compiling it I got bored and opened htop and saw that only even numbered threads are doing most of the work as seen in the screenshot below
Well yes, totally. Not everything in a dependency graph can be parallelized, only those crates which do not depend on each other can be built in parallel. If there are fewer such crates than CPUs/cores, then you won't see all CPUs being used.