I'll shill caniupgrade.flights which has info on which flights can be upgraded using various instruments. I need to update it for PlusPoints, but otherwise it's accurate.
Your comment is warranted, but I think I know at least one contributing factor (normally fly around 200K / yr on United)
I'm making up an extreme case to make the point, but let's say the plane's business / first cabin is 100% empty / unbooked, and the economy cabin is full (even overbooked) and the standby list is 30 deep.
Even though UA is reluctant to give out free INT upgrades, if it means they can put 30 more people on that plane and collect the revenue, they'll do it. There's more to it than that, I'm sure there's a heuristic, in fact one day I hope there will be an AMA "I code the conditional logic for airline XYZ's algorithm <for Global Services> <for INT upgrades> <for domestic upgrades>" but until that time, we can only discuss what we've observed and speculate on the rationale.
Me, I'm still wondering why when I'm first on the ugprade list and there are 8 unsold first class seats, why do I have to board (in my economy seat), settle in, unpack and decompress before some gate agent comes waddling down the aisle 30 seconds before the door closes to tell me I've been upgraded. It was clear 30 minutes ago that was the case. First world problems, I know.
No idea then - but it's happened at bag-drop at least twice. Just after I achieved status each time I think so I thought it was a thing they did to make you think it was really worth having achieved it.
I've gotten it once (HND to SFO), so it's not unheard of, it's just not official policy. 24 hours before the flight departs, the upgrade list gets transferred from the online waitlist to the airport, and check-in staff and gate agents have full control at that point (this is why UA doesn't let you use upgrades within a 24 hour window). Occasionally, you can get someone at the airport to put the upgrade through (though I've found it burns goodwill, so don't do it too often).
You won't get upgraded if it's a full flight and most biz seats are paid for, even with GPU. With gold especially unlikely, that will put you in the middle of a usually 50 person waitlist for 3 or 4 available seats.
When I fly from SFO to Canada I get upgraded pretty regularly on gold status. Of course these are domestic first, so while better they still aren’t great.
I'll shill caniupgrade.flights which has info on which flights can be upgraded using various instruments. I need to update it for PlusPoints, but otherwise it's accurate.