Competition. Someone is highly likely to figure out how to shave costs. Then the company can't even sell the thing and the people lose their jobs.

The point of an hourly wage is that it's a contract to be paid some hourly amount regardless of how many things are sold. The company bears most of the risk. Sales are always dynamic. So how can the company pay the employees for every widget made if the things they make aren't selling for a price that covers the cost of paying the employees?

Any thing created will never sell consistently and never sell forever. So again, skill must change. Marketable skills are always changing. During tech change, the price and demand of the old product drops.

From 1900 to 1920 millions of people lost their jobs to cars. They spent their entire lives around horses. Leather work, carriages, blacksmiths, farm equipment, etc. In just 20 years the horse and carriage was toast. Everyone had to reskill for cars and other jobs because cars took fewer people to make than trending to all the horse stuff.

A modern example is computers. Until the 80s and 90s there were huge work forces processing everything with paper. It wasn't just those workers that had to reskill. The paper mills had to reduce output. Fewer printing houses. Fewer printing press repairmen. Fewer parts manufacturers for the presses. Less ink. Less forestry management for paper. And so on.