Why are TV screens rectangular and not square?

The earliest CRT displays were in fact round. However, that was more due to manufacturing tradeoffs of making large glass envelopes with vacuum inside than a desire for the display area to be round. Rectangular makes more sense than round for a variety of reasons: It’s what people want to look at.

When did TV Stop being square?

It wasn’t until the early 2000s that 16:9 (1.78) televisions hit the market in masses, and changed the aspect ratio game forever. No longer was widescreen a format only for film, but now it was a television format too.