I think it's a US thing. I've never not got the seats I'd reserved either.
I've only done two domestic flights inside the US, one from JFK to Miami, and one from Minneapolis to Seattle, and both of them were utterly awful when compared to intra-europe flights I've done (so similar distances).
I've only done two domestic flights inside the US, one from JFK to Miami, and one from Minneapolis to Seattle, and both of them were utterly awful when compared to intra-europe flights I've done (so similar distances).