Why would installers deal with cheap Chinese devices? Every installer I’ve dealt with has a relatively small selection of vendors and devices that they know and trust. They aren’t reselling random crap from Amazon.

No, they're each having a relatively small selection of vendors re-selling white-label Chinese crap.

In my case: local A/C installers are reselling Haier aircons. Good hardware. Shit software (see "tale of the two apps" elsewhere in this thread). Or, a nice local company selling floor heating solutions. Got electric floor heating from them. Control panels have "smart home integration". Guess who made that? Tuya. The world's finest seller of white-label "smart" devices. Complete with a shit app.

Home Assistant is the only thing that makes the two device classes more convenient remotely than through an IR remote or using the wired-in control panels.

Seems like a professional could just limit their install to known reliable devices?

I've had plenty of trouble with Home Assistant upgrades randomly breaking smart devices from well known vendors, e.g. whenever I update I dread to find out whether my Neato vacuum robot is still supported or whether that integration has crapped out again. Same for my LG smart tv and several other devices. I love Home Assistant but it's not exactly rock solid...

Right there with you that the amount of customer support needed for such a service kills the incentive. I already hate having to fix my house when something breaks or a battery dies. The last thing I want is Joe Schmoe calling me up at 2am because his power went out and shorted the home server, and he has no backups because he disabled the ones I set them up with while "tinkering"