My friend is in the process of completing his residency in emergency medicine. When I asked him what the most difficult part of his job was, he told me, "having to see one obese person after another, knowing what I tell them will almost certainly have no impact."

Lots of people have already mentioned the incentive factor (well covered by Bill Gates in his comment about baldness: http://www.ted.com/talks/bill_gates_unplugged.html), so I won't speak about that. Rather, it might be worth thinking what solving these real problems involves.

Fixing obesity? Working with obese people day in and day out, trying solution after solution with no success? It's not just that these things aren't glorious - but they're also soul crushing. When you make a social network, people around you get excited for you. When you make a weight-loss application, well... not much happens.

For an industry that is already prone to depression and mental illness, it's not entirely difficult (although it may not be right) to see why things have developed the way they have.