Programmers love to oversimplify things; "do easily with an RPi and some simple Python" is kinda meaningless. Like, yes, an RPi is a general purpose computer and Python is turing complete, thanks.

For one, UI/UX is actually hugely important for a consumer device and definitely nontrivial, but on top of that, there is way more that goes into creating custom hardware than a bill of materials (which isn't just saying "Raspberry Pi") and choosing a programming language...

A thermostat is controlling a very expensive device that runs on a highly flammable gas that costs me real money to use. I want 0 serious bugs. I also want 100% uptime. A poorly made "smart thermostat" is way worse then the old school analog metallic ones imo. I also want my partner to be able to control the temperature in the house. These devices are actually not simple at all and I assume that's the reason there isn't a good open source/open hardware solution.

Embedded systems aren't some mystical impossible thing - contrary to the previous commenter I actually find working with them easier then designing GUIs - but the commercially available devices are definitely nontrivial to recreate