Trades is a good example. Plumbers and electricians are generally paid quite well. Specialist industrial trades, welders, etc. can be paid amazingly well.
Although so can grass cutters, pool boys and painters and they seem pretty unskilled to me.
Sure, you can mow a lawn, but how many lawns can you mow in a day and still do a good job? Those guys are skilled at maximizing the efficiency of the business. If you took a good existing crew and replaced them with all fresh people with no experience, the business would probably fail pretty quickly if you don't make enough money to pay for their labor.