the inability to claim an instant upgrade "instantly" is because the ticketing process is not actually instantaneous. the ticket can take up to 10 minutes to actually issue after you buy it, but usually it's closer to 10-60 seconds. you can't get it upgraded until the ticket number issues, despite the fact that the PNR issues immediately.
back in the days of the UA/CO merger, some tickets were actually taking hours or days to issue, even when they were 100% on UA metal.