This is common for many real time applications, not just games. It can be a result of designing for WCET (Worst case execution time). By always running on worst case even if not needed, your testing becomes much easier because there is less need to go into every corner case and see if your frame rate drops. For gaming its less of a safety issue but it gives a more consistent experience.

Games also really don't care since they are single use applications. You are not going to run something else cpu intensive in the background while playing.