You seem to be ignoring price, and refuting evidence of the price in your comments throughout here.
While there may be $500 one-way tickets from NYC, NYC is on the tip of the US closest to Europe. There's no way you can reasonably assume all Americans would use NYC as their gateway to Europe, or could do so in a timely and cheap manner. Going from DFW -> Frankfurt in July is showing $1,700 round trip per-person. If my SO and I are going to spend $3,400 getting to Europe, then I would want to see as much as I possibly could - which means a packed itinerary that's prepared well in advance.
And even $1,700 in the US is being pretty generous. For people who don't live near international airports, the flights to those airports can cost hundreds more. A puddle jumper from my hometown in Wyoming to Denver International costs $500 each way per person(The alternative is a day of driving each way, so that's your vacation time - 2 days right there). So for 2 people you're losing 4 vacation days driving or $2,000 flying. This is not something you ignore when planning a vacation!
Friendly Ghost mentioned in the comments that round-trip tickets from Indonesia cost $2,000 and you retorted with the cost of a one-way trip being $800 to Singapore and $100 from Singapore to Indonesia. So, round-trip, based on your numbers, is $1,800(I'm not sure your retort proved what you thought it proved). Given that the cost of flights can change wildly throughout the year, a difference of $200 isn't surprising. Also, someone in Indonesia who wants to go to Europe isn't going to think "Ok, the tickets are going to cost $2,000....that's just too rich for my blood. BUT...if they only costs $1,800...that's perfect!" These are just the tickets to get there, if $200 is really cramping your budget that much, it's probably best to continue saving and go later.
I see the above as an addendum to David Mulder's answer. You just seem to be ignoring the price like it's nothing, when the real price for a lot of Americans is going to be in the thousands just for getting there.
There's also likely a selection bias, and it's two-fold.
The Americans you're more likely to see in Europe going from one place to the next are going to hit the huge tourist attractions. So, if you mostly notice Americans in these areas - they're necessarily more likely to be the type on a tight schedule. They're not the kind of tourist to go off the beaten path and explore. Some of them will, but not all of them. Then, the ones who do explore are going to be spread across all of Europe. What are the chances you bump into one of them randomly?
The additional selection bias is that people who want to do rest and relaxation are less likely to go to Europe to do it. There are plenty of places in Florida, Texas, Southern California, Mexico, and the Caribbean that allow for R&R. I could do two weeks of cruises in the Caribbean for less than the flights to Europe.