Because they don’t want to have to work that hard for their money. They get more value for their effort by focusing on something else instead of trying to maximize profit for a single business.

The demand curve for a business could be such that you sell 100 widgets and get $100 profit, sell 200 widgets get $101 profit. Why bother trying to sell more than 100 widgets?

Maybe you can sell 50 widgets and make $90, that sounds like a pretty good deal. Instead of making 50 more widgets you could do something else and make more than $10 profit in the same time.