Just think of it like one of those Black Friday sales where they advertise they have cheap or free merchandise. However, if you actually go there, you find out they only had one.

UA advertises this upgrade benefit (GPU, RPU, CPU, Miles, etc) however they don't advertise the actual numbers. Sure there is R availability, but you have to be really flexible and/or lucky to actually find it in advance. In some ways, upgrading at the gate is more "fair" in that the upgrade is based on the list which is ranked in some way. When you apply an instrument at booking, you are grabbing that upgrade where perhaps some higher paying or higher status passenger is missing out.

For me, much of my intl travel is leisure so I don't book an intl flight unless R is available and am willing to set my dates, duration, routing around that. Some must travel on certain dates and dont have that luxury.