There are other opportunity costs other than direct monetary income.

For example, to mow your lawn, you have to find a time to do it every couple of weeks when the weather cooperates, be at home at that time, store and maintain the equipment, have a pair of "grass shoes", clean up afterwards, etc.

This might be worth the effort if you don't have much disposable income. But if you have money to blow, hiring someone to mow your lawn can give you more time to do something else you'd rather do.