Because HVAC in theory should outlive most other standards that have come and gone. Its totally fine to built on top of it, but using basic hard wiring to turn on/off is essential - even if just as a fallback. in 20 years the hvac should still be working, but will the wifi spec it shipped with? What about matter/thread?

Theres certainly a middleground between "must use existing old ass 2 wire setups" and "requires fancy always on cloud subscription"

Surely we could come up with a device that might require an electrician to run a few extra wires (or maybe even larger wires or something) and still run perfectly fine offline, for as long as the hardware functions, with some type of integrated manual controller. And of course this device could also have an internet option and an app and wifi for all of the fancy stuff.