[edraferi]

This is why some governments raise speed limits. Faster traffic -> more energy required per mile -> more gas consumed -> more gas tax revenue

[selectodude]

Or it’s because it’s politically popular to raise speed limits since they usually raise them to the speed that is almost, but still below, the speed that everybody is already driving at currently.

[WorldMaker]

Also in the US the AAA heavily invests in lobbying for higher speed limits every year. The AAA isn't quite the villains in American politics that the NRA are (ETA: or at least, have become), but they are historically close. It's easy to forget that AAA was founded to fight speed limits in the first place.