It's not because nomad life is so great, often it's because they struggled because--again--life in developed areas is tough and so they opted for the easier way.
Moreover, those have still some home base in an developed country, a back door where they can always settle again, so it's not really nomad, it's just a long vacation labeled as something adventurous.
For us from developed countries, it's so easy to take our backpack and travel dozens of countries and just spent a fraction of what we would spent in NY, London or Berlin. You know it's really nothing special or something people should admire. Everyone from a developed country can do this. But nobody from not so developed countries could and want to do this. I personally would miss a goal and again the challenge. I can just buy a ticket in 5 minutes and go, find some remote work and live for peanuts. Where is the problem, everyone can do this There is no risk and no goal. And because your FB friends got tired of all your useless and boring nomad posts you need to write now public blog posts and spam social news sites telling people that what you do is great justifying your goal-less endeavour to yourself.
So, another popular reason of being a 'nomad' might be: I am struggling with the high competition in my current environment, want to save money or spent a fraction in an underdeveloped country and live like a king. But not like I want to know who I am (maybe they know it afterwards but I think it's rather unrelated).