It sure isn’t Microsoft, Sony, Google, Samsung et al.
They are not even trying hard to make it look like they are above ads revenue streams. Which is fine, as you mention everybody else is doing it in some way, but lets be honest here.
If it is not so good, what's the point?
There are also many regulations and market issues with making a panel TV, everything from standby power regulations, the tuner type (ATSC/DVB, etc.), warranty, accessibility requirements, etc. And even more issues with the ASICs that run TVs from companies like Amlogic/MediaTek/RealTek; Apple would either have to make a deal or build their own silicon IP for things like ATSC/DVB decoding, image scaling, audio decoding and processing (i.e. Dolby). And ecosystem issues like HDMI CEC, HDCP, etc.
If you side-step all of those issues and just bond a really nice display to an AppleTV and have no tuners, then you probably aren't adding much value beyond buying an LG/Sony and and AppleTV separately.