(Ignoring the high likelihood this is a trolling question)
The simple answer is because the overwhelming majority of TV content is shot and captured for presentation on a rectangular screen.
There’s no value in having a circular screen if 99.9% of the content only appears in a rectangular spaced section of the screen.
That said there was a bit of a phase with funky shaped TVs some of which were round (e.g. [this one](https://www.retrotogo.com/2011/12/ebay-watch-1960s-space-age-keracolor-circular-television.html))
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