Because eventually all the tasks that a normal human can do can be done cheaper by a machine. Whenever more automation enabled economic growth in the past, a (large) part of the new economic activity had to be done by humans. We were lucky that the growth was big enough that more new jobs were created than old jobs were lost. In the near future the fraction of new tasks that can be done by humans will shrink. It is plausible that economic growth as a whole will shrink too.

And of course there are the short term effects to consider too. If truck driving is automated, a fifty year old truck driver is unlikely to find a similar job. Changing careers gets progressively harder as you grow older. Many people will be unemployed for the rest of their lives despite other new jobs being created.