You underestimate how hard it is to switch a field. If it takes $50k to switch I will get better margins but first I have to have $50k. Then how do I know that new thing I will do will bring that profit in first year? That is already $50k and lost profits for figuring out new market.

If you look at risks involved and potential loss of money instead of only looking at higher margins, it is easy to see why people would rather earn their 5%.