Because you will still have to compete for desirable jobs.

For menial labor, most of it can/will be automated.

For undesirable jobs that can't be automated away, employers will have to make the jobs desirable. One way to do that will be to increase the pay, but, that is not the only means for doing so. There are many things employers can do to make employment more desirable that have a low or zero net cost. I'd even argue that some of those things would save money/resources in varying amounts even absent the pressure to do so.

One very good and interesting effect that I predict is that individuals whose employment places them in ethical dilemmas will no longer have as much pressure for compliance with potentially unethical demands, since the entirety of their livelihood no longer depends upon compliance.