It also depends on the system.

Back in the day running something like Super Mario 3 often meant composite video going through RF hookup and the TV "recovering" and decoding. I'm sure that accounted for a bit of the blurriness.

Contrast this with a game like Commander Keen for DOS running in 16-color graphics on a VGA CRT monitor which sends separate R, G, B signals. I remember Keen being very crisp even as a kid who practically pressed their face against the glass when playing.