Most of it's been said already, but let me add my 2 euro. It's definitely not a race thing. The ironic thing is that it's only your inexperience with the "phenomenon" that makes you think only "whites" do it.
If by "whites," you mean "American," then you're dead wrong. It's literally almost a national pastime for Aussie's to take their Gap Year and travel after high school or college. Same with Israelis. After fulfilling their mandatory military duty, 20-somethings head out for a year to see the world and escape the tension they've lived their lives under. It happens everywhere. Every hostel I stayed at, I met multiple "whites" of a different ethnicity. Surprisingly, or not, most all of them were fluent, if not functional, in English.
If by "whites," you actually mean white-skinned caucasians, then you're wrong again. See the comment about Israelis above, plus countless Asians and South Americans. I'll give you that I didn't see many black backpackers in Europe. But to say they were all white is just preposterous.
As for the concept of "why," that's difficult to explain. I hate to go down this road, but it's one of those things where if you have to ask, you won't understand. Believe me, I'm about the least pretentious person you'll meet. And I do my best to not let my lack of pretension become its own pretension. But I do know what you mean by the type of people you are describing your friends as. Love that Pulp song. The thing about backpacking is that, like ANYTHING ELSE IN THIS WORLD, it takes all types.
Plenty of people go backpacking for the Wild On! experience. Ever seen "Rules of Attraction?" Like Kip Pardue's European trip. And even still, to each his own. I, personally, backpacked partially to get in touch with my roots. Both of my parents are immigrants, and I visited their homes for the first time in my life. I spent about 2 weeks doing that.
And then I spent the other 2 weeks seeing buildings that were built 2000 years ago, works of art by the masters of the craft, talking to people from all over the world about the things they care and worry about, and generally having the best time of my life. Believe it or not, most of the idealist hippie liberals that travel do care about their fellow man, and we could all stand to learn a little about eachother. This thread probably wouldn't have happened if that happened more often.
Oh, and sometimes it is just all about waking up in a co-ed hostel next to Brazilian girls: