SFO has a 45 minute check in cut-off time due to baggage check restrictions.

If you haven't checked in before the 45 minute cut-off, the computer will deny you and the check-in has to be processed manually. If you don't have bags, an agent will usually do it for you, but if you do have bags then you're SOL.

At 30 minutes before the flight, control has already passed to the gate and there's nothing a check-in agent can realistically do in terms of that specific flight.

I did once do an EXTREMELY last minute booking on UA (left the house less than an hour before the flight) and upon arrival at SFO, there was not much the ticketing agent could do aside from book me on a different flight and slap a standby card into my hand.

I did get on the flight I wanted, but only because I didn't have bags *and* because I cleared the standby list. The ticketing agent was helpful, but had to work within the United rules. I presented an option that allowed her to help me, and she did. Had I simply demanded that she sell me a ticket and get me a seat on a flight that was leaving in less than an hour I have no doubt I would have been shown the door.