Why? Because the UAW doesn't just work for higher wages and better working conditions, it completely takes over the operation of the firm, decided when and how anything is produced.
I'll give the reader a personal story to illustrate this. About 8 years ago I was attending a pre-delivery run off of a large piece of industrial equipment. The techs on the manufacturer's assembly floor were spitting mad about the machine they had just delivered to the UAW at Allison Transmission. This machine replaced an older piece of equipment, but they had to reprogram it to match the much slower rate of production as the older piece. The maddening part was both machines were robotically attended. The UAW's complaint was the new robot was working too fast.
Watch the great old English movie “I’m All Right Jack” from 1959. One of the rare films that shows the ridiculousness of unions.