Because one feels that monetary gratification is more than gratification one gets for acquiring knowledge. One feels that after more than 16+ years of schooling it is time to take a break, and during that break one finds that 1) the marginal utility for acquiring knowledge has diminished to non-existent, and 2) the marginal benefit for further knowledge is less than the current monetary reward for working.
Then again I am talking about other people. I like studying and enjoy knowledge acquisition equally with monetary reward (so the best solution is really working/studying parttime)
Again that's just me....
