Why can't one instance of resource-intensive code not completely load the CPU?

Question

C++ / C#

Regardless of the complexity of the calculations and the number of threads, I can not take 100% of the CPU with one instance of the program. But, if you run 2-3 instances, then the CPU can be made to work at full capacity.
Question: how to transfer the authority to the program to use all processor resources at 100%?

Answer 1

Does the program load anything other than the CPU? Maybe I/O is interfering? And so - to raise the priority and there will be more CPU.

Answer 2

limit on devouring the processor per process is not worth it?

Answer 3

The easiest way to figure it out is with the Concurrency Visualizer