Yes, other countries have visa requirements; I myself was bitten by the Schengen Zone requirement last time I was in Belgium and Germany. But the point is that I don't need to spend hundreds of dollars, navigate a series of confusing bureaucratic hurdles, and then have my body and bags x-rayed and the minutiae of my personal life cross-examined by high-school graduates on a power trip just to have a 4-hour layover in most other countries... unlike America.
To contrast this, the last international flight I took was entering Mexico last month (where I am now); the line was long, but I wasn't asked a single question before my passport was stamped. The 2nd to last international landing I had was in Belgium: I was asked the purpose of my visit and was waved along in 30 seconds.
The most complicated visa requirement I've run in to was Vietnam, which required a stamped visa to be processed before you crossed the border. Even then it was a matter of giving $25 to my travel agent along with a short customs-style form.
Yes you can say that, say, an Iraqi citizen will have a hard time getting in to any country, not just the US; but when we're talking people from England, Europe, China, hell, even Mexico and Canada--neighbors and close allies for generations--the process can still be harrowing. And that's not just unfriendly, it's damaging to relations in the long-term. 20-something backpackers who get hassled crossing into America today, or who choose to go to a different country for 6 months instead of face American border bureaucracy, will take those experiences with them when they enter industry and leadership positions in the future.
Edit: another anecdote--on entering New Zealand I was asked for proof of onward travel (as I later found out, a requirement to enter the country). I didn't have any; I had planned on staying a few months then carrying on to Australia when I felt like it. When I told the agent this, she laughed and waved me on. Can you imagine a US border agent doing that?