Each application can opt in or out of full screen mode. From Apple's perspective, Activity Monitor is an "instrument", like a calculator or a multimeter, for example. You can test it on your machine - Calculator app also refuses to go full screen because it is just an instrument.
So the devs concluded that Activity Monitor is not something that users would want in full screen. Users would rather want it on the side while they are focused on the actual program whose CPU/Memory/Disk/Network activity they want to keep an eye on.
The section of human interface guidelines on design for full screen mode has this guidance:
Enable full-screen windows only when it makes sense. Full-screen mode is useful when a user wants to be immersed in a task, but not all apps require immersion. For example, some utility apps like Calculator work better with smaller windows that reside onscreen alongside other apps. When porting an iOS app to macOS, don’t assume the app must support full-screen mode in macOS
Perhaps, the utility aspect of this is prioritized so the menu bar isn’t hidden for people that wish to be immersed in the activity display.