Why screen saver is called screen saver not anything else?

Probably because back in the day everyone used CRT monitors. These monitors had a problem that if one image was shown on the screen for a long period of time, like data tables, they would be "burned" into the screen and mess it up. Therefore they started using a moving graphic that would turn on after a certain period of time and "save" the screen from being damaged. Hence the name screen saver. The same thing happened with the idea of a computer bug. The first bug was in fact a moth that got stuck in one of the first computers causing a short, it was a moth. So the names like screen saver and bug have stayed with computers to this day.

Beacuse it "saves" your screen from being burnt out by showing one constant picture.