Lack of purpose I suppose. If I have enough money to pay rent, get trashed with the boys on the weekend, and buy cool things every now and then, why lift more than I need to at work?
When my dad was my age he just recently bought a house, got married, had a kid on the way. His effort in growing his small business made tons of sense. For me, if it wasn't for my personal drive to be great at my job - a job which I have a passion for (and I know luck has a lot to do with that) - why would a try harder than I need to? I'm not really responsible for anything, you know?