Blame the Department of Transportation (emphases mine):
If you’re involuntarily denied boarding, the Department of Transportation regulates what you’re entitled to. Here are the rules, as published by the DOT …
… If the substitute transportation is scheduled to get you to your destination more than two hours later (four hours internationally), or if the airline does not make any substitute travel arrangements for you, the compensation doubles (400% of your one-way fare, $1350 maximum)….
The US government sets the maximum compensation amount. I suspect that amount was set years ago and hasn’t changed
The article I found this in calls $1,350 “sizable”. No. That is not “sizable". I was almost bumped myself because Delta couldn’t find takers of a similar offer. $1,350 was a reasonable offer in 1990. In our 2017 lives a reasonable offer starts at $5,000. If there has to be a limit that limit should be around $10,000 — regardless of ticket price.
So, yeah, I hate United as much as most travelers. But this one the Feds own.