It’s not. Upgrades out of SEA are rare on UA, and a Platinum CPU is behind all Global Services or 1K members, Platinums on a higher fare class, and anybody who used an instrument. You won’t find as many of those in SEA as you will in SFO, but what you will find is that people are willing to buy F in the first place or pay the upgrade fee, particularly on the longer flights (IAD / EWR / IAH / ORD). For example, this coming week, Thursday or Friday, no CPUs would have run yet. Thursday, the SEA-EWR flights are currently showing 1 seat, 4 seats, 5 seats, and 5 seats available in J — getting even one upgrade for a Platinum would be a surprise, let alone five. Friday is worse; each flight has 2 seats left. I suspect nobody will get a CPU on any Friday flight, and I suspect there won’t be more than two CPUs among the 15 seats available on Thursday either. They’ll sell most of the rest, either for regular price, as part of a buy-up at check-in. If there are a couple left over, they’ll go to people who have used upgrade instruments on that 5-hour flight.

That’s not to say that you never have a chance, but the primary benefit of Platinum is free E+ for yourself and 8 companions in an E+ cabin that’s generally large enough where that’s a real benefit. Any CPUs you might receive should be considered a welcome, but unexpected bonus.