Yup, that would be the ideal solution, but their prices skyrocket as soon as one goes above the "normal" office/gamer panel sizes. They're also probably built using better parts and better quality control. Economies of scale do the rest.
I looked at monitors 35" and larger and the cheapest I could find are at least twice as expensive as the corresponding TVs.
Maybe it's just a marketing thing but there are definitely differences which make monitors a hard sell to be used in place of TVs (e.g. in a living room, opposite to the sofa).
Not readily available in the sizes popularly used for TV/video viewing, as far as I can tell. Large-format monitors tend to be full of "digital signage" or touchscreen features now; some are even Android-based.
Same for speakers. I have a decent sound system already, i don't want to pay a extra $20-$50 for crappy speakers that i will never use. I want the best panel for my $, nothing else.
A $50 player box does everything better than the built-in player hardware and software.