CPU cores don't really "wear out" by use. The only thing that is really dangerous is badly overheating the CPU, but on modern CPU's internal safeties kick in which will throttle the performance before that happens.
A CPU is designed to be able to run briefly on full load (all cores) without overheating. After that it starts throttling back its performance and (if still too hot) it will shutdown to prevent damage.
So, no matter what the load-profile is, you can't really hurt the CPU just by driving up the load on the CPU. And certainly not using just a single core.
And NOT seeing any load on all cores not really unusual.
Getting all cores to do work simultaneously is pretty hard on a CPU with that many cores anyway (outside special benchmark programs that are explicitly designed to stress all cores at least).
You either need to run a whole lot of programs in a parallel or run a program that needs many internal parallel processing threads. Usually a PC just doesn't hit that many parallel processes.
64-core CPU's are usually only found in servers and very high-end workstations. They are usually total over-kill for a normal system.
I've got several 12 and 16 core systems at my disposal at work and at home. I rarely get them to use all my cores with normal work.
Only happens if I have to compile a very large software project (multi-threaded build) and have several heavy multi-threaded applications (3D rendering software, video trans-coding) running in the background as well.