> What's the actual steel man argument for why noncompetes are good?

It probably depends on the kind of job.

If say Walmart tried to use a noncompete to stop cashiers from going to Target there probably is no reasonable argument in favor of that.

On the other when the employee is a top level executive who knows all the company's trade secrets and all their plans for the next year or so and they want to go to a direct competitor it is hard to see how they won't use that information at the competitor. Even if they scrupulously try to uphold any NDAs they are under and so don't consciously do it stuff will leak.

If the first company sues accusing the second company and/or ex-employee of using such information it can get pretty messy, and consumer judicial resources better used for other things.

A policy then of allowing noncompetes in this situation might overall be beneficial. Top level executives are generally well compensated and should be sufficiently sophisticated financially to understand the consequences of a noncompete and take that into account when deciding on taking the job so having to sit out 6-12 months before taking a directly competing job should not be a serious issue.