It also costs a lot of money to reconfigure an aircraft, as it needs to go in for service, spend a few days out of service and not making any revenue. Plus, while they are taking the seats out, it may be a cost-advantage to do a D-check at the same time, so if it is scheduled to do this in the relatively near future, may not be in UA's financial interest to rip out seats, replace them, and then rip out the seats a few months later.

I'm sure UA has done some sort of analysis on the most cost-efficient way to re-configure the planes, probably with a lot more info than you're non-ability to book an F seat at the moment. I don't have any inside info, but just a thought.