I'd be surprised if anyone who regularly travels to the US hasn't found it to be extremely unpleasant at times. A while back I spent 6 months or so going back and forth from the UK to Houston or Atlanta, and every time I absolutely hated going through immigration. I've travelled to dozens of countries, and literally everywhere I've found the border guards to be friendly - except the US. Every time they behaved like power-hungry bullies, or I saw them behaving that way towards others. One time I travelled with a female colleague who was basically harassed by ridiculous questions from a very angry and loud border guard for no reason at all. He wasn't satisfied until she was in tears.

Another time there was a really long queue after getting off a flight (think it took over an hour to get to the desk), and there was a pregnant woman in the queue who seemed unwell and wanted to sit down - a guard shouted at her to stay in line, refused to get her a chair, and refused to let her jump forward in the queue (despite prompting from willing fellow passengers).

So the reason people don't want to travel to the US? Because it's a fucking horrible experience.