Why portable split air conditioners not common? What’s the catch

I can think of several reasons.

1. A thermostat is a reliable, proven, simple, and inexpensive device which works well enough in most cases.

2. A humidistat and its derivatives rely on a comparatively expensive relative humidity sensor plus some finicky electronics. A thermostat would still be necessary to prevent potentially dangerous results, like shutting off when the humidity is below 75% but it is still 40°C/104°F (admittedly unlikely), but also the humidity threshold may not be satisfied until the temperature is 5°C/41°F. Not so good for many living things.

3. Humidistat technology of old has a limited range, limited responsiveness, and a short lifetime of it being accurate. Stable and reliable sensors have been around only since the 1990s.

In short, it is reasonable to expect new devices to commonly incorporate this functionality. As manufacturing volume increases, it reduces the price and consumers expect more.

I'm in Michigan and we get harsh winters and salt our roads. Salty roads and ice get on our cars, and this combination is really harsh on our metal parts of our cars. Salty humidity is also very harsh on any metal parts, even galvanized steel. Now galvanized steel in the US has improved in the past 20 years, but it is not impervious to salt water.

That said, you will need to have all important parts coated some how to prevent corrosion as salty humidity is super corrosive. There is a service in the US that coats cell phones with a nano coating. They don't even take apart the cell phone. After the phone is processed you can drop it in a pool and it will still work. The phone is guaranteed for life vs moisture and water. Liquipel.

If the AC company has this service available they can coat their PCB boards and that should help a lot. Or they can use another coating on the PCB boards. The coating cannot conduct electricity or it will short out the boards.