Because it's an extra expense to support it with basically no value. The reason to give a TV a network connection would be to use the applications built into the TV... ones that you don't need if you're using an Apple TV.

And hell, I can't even find any evidence that there's TVs that support HEC.

The value comes from the value of selling ads and knowing everything about the product. In this case the viewer. Value prop for building the ability to still have insight into your product is there. It captures all the people who think they are clever by not connecting through WiFi and not understanding that it can be done over HDMI.

There's no value _to Apple_ (in this scenario) to building ethernet-over-HDMI to let your TV sell you ads, because Apple isn't seeing a cent of that.