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

It makes it possible to confidently buy a business that's mostly or all goodwill. Otherwise the previous owner can simply poach all the clients.

Also lots of jurisdictions allow non-competes as long as the employee is paid for the duration of the non-compete clause. Obvious win there: paid vacation or double up your salary by working for a non-competing firm.

Non-competes on employees without compensation are obviously bad.