I think it's because recent trends (in computing power and costs) have made it very profitable to chase after me-too apps (photo-sharing being the poster child for this trend) and people are simply going where the money is instead of working on really hard problems that will likely never make a return on investment.
I can see both sides of the argument (i.e. whether it's OK that this is happening)
1) What the Wright brothers worked on meant that humans could fly for the first time ever. What Alexander Graham Bell worked on meant that humans could talk to each other over large distances for the first time ever. What the early PC/Mac pioneers worked on meant that huge computing power was put in the hands of the average person for the first time ever. These were history changers
I think many would agree that working on a project similar to one of the above is on a very different level, and much more satisfying, than working in a startup that is producing yet another photo sharing app.
What the OP may be lamenting is that so many bright minds are being "wasted" on yet another Groupon clone where no technical breakthroughs are being made, and not on useful projects where technical breakthroughs are needed, like renewable energy, natural language processing, etc. Whoever solves these problems will enable something that was not possible before and have an impact on the course of humanity.
2) On the other hand, there is nothing wrong with wanting to make money, and wanting to make it in the most easy way possible. If so many other professions can make a lot of money without really benefiting humanity, why shouldn't engineers feel free to exploit the recent gold rush in their field and make as much money as possible, with little regard to how useful their product is to the world at large?
Finally, I wrote about this a couple years ago: http://www.startuptrekking.com/2010/09/web-startups-are-just... ["Web startups are just attempts at social engineering"]