That may be a small part but a much bigger factor is price differentiation and market segmentation. A 55" monitor is pretty extreme - any gamer who wants that is going to be willing to pay for it. A 55" TV is perfectly normal though and bought by normal people.

Same reason nVidia's AI cards are so expensive even though they're very similar to their graphics cards. Or "audiophile" speaker cables are expensive even though they're identical to cheap ones.

Product prices are partly a function of the price people are willing to pay (demand).