it's 2017

Probably because it's speakers aren't (usually) great, may not be stereo and would require the TV have a wireless system that can work with the phone? (and the phone have the ability to act as a wireless speaker).
The number of people that would use it would be relatively small as well so not a major selling point.

true, but you could then at least use the phone as the equivalent of a bluetooth dongle.

it's not so much a case of why do it, but why it hasn't been done, would be convenient for all sorts of devices not just specifically phones/tv's

for example the ps4's capability of using the controller for audio output, albeit much more universal.
 
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