Color monitors were expensive in the 80s, so it was common for PC users at the time to use a TV instead.

Hell, even today I use a TV as a monitor (LG C1). They make TVs specifically for gaming now, and they've improved quite a bit in the past 5 years to the point where they're comparable to gaming monitors in terms of input lag, response time, and all the other areas where it matters for games. The only category they fall behind in is bandwidth, due to the limitations of HDMI (and unfortunately no gaming TV supports DisplayPort yet), but even then most people are perfectly happy with 144Hz @ 4K, so it's not the end of the world.