Automation typically costs millions to implement the first time, but then replaces labor that is paid every year. So you lose money the first 5 years, and after that it is a lot cheaper.
Don't forget that technology marches on. When GM started automation in the 1960s it was doing a lot of design work on it in house, so it was several million $ to replace a few people making $10k/year. Now you can buy CNC machines to make many of the parts off the shelf, and those machines directly hook up to the CAD your engineers are designing parts in, so automation is cheaper in just days (and the machine probably holds tighter tolerances for more quality). Even today though there are a lot of things that are we could automate, but it isn't practical as the custom machines needed would be more cost than doing the work manually - but there are less and less such things all the time.