It's just unnecessary complexity and another thing that can go wrong. I know when I'm using it and not using it. I don't need the device to guess. The device guessing doesn't actually improve my life in any way.

And it's going to guess wrong a lot of the time. Automatically turning on and off both have unsafe failure modes that lead to it being on unexpectedly (it turns back on when I don't expect it to, and it doesn't turn off when I do expect it to) based on imperfect sensing hardware and software that can both stop working, and I'm not ok with unsafe failure modes in a device that will burn down my house.

This is also the reason that I disfavor battery-powered soldering irons in general, but at least being portable adds something you may need and can't otherwise achieve.