The problem is there isn't really much of a market for something like that. Sure, you can have detailed documentation on such a product that explains how it's functioning, why it might drop out when your ip changes, etc., but who is going to understand that? People like you and me, yes, but we're a pretty small market.

In order for there to be a reasonable audience for a product, you have to target the least knowledgeable person - or at least someone pretty far south of the median. Those people aren't going to understand what's happening, and they're going to get upset each time their ip resets and they can't connect to their thermostat anymore. They're going to take the frustration to the thermostat company, they're not going to understand what the service rep tells them, and they're going to return the product, leave a bad review, and buy a nest.

No one wants to be in the position of making and selling such a product. That's why you pretty much need to make your own with an esp8266. And that's why so many of these IoT devices end up being far too bloated.