This has been true for years since 2014 - Changes to MileagePlus Award and Upgrade Policies - Eff. 3-Feb-2014. While it was a change in 2014 that many were not happy with , there is some logic to it, as it probably costs UA more to acquire seats on partners vs transporting you on their aircraft.

And as you said, you have other options, just not as convenient.

But this is dead issue and obviously UA is not going to change that policy -- especially now, See the reference thread for a extensive discussion. It is off topic for this thread.