> I feel like the only explanation here is that the smartphone duopoly has worked hard to condition customers into accepting this.

This is older than the current smartphone duopoly; IIRC, with a few exceptions like the N900 and the Palm Treo, cell phones were always very locked down. It probably came initially from the phone companies being control freaks, and wanting to restrict which devices (even mechanical ones with no electrical or radio interface) could be attached to their phone system.