I agree that it's short-sighted, not only in the way you describe, but also in a much more direct way. While noncompetes make it easier to retain existing employees, and pay them less, by the same token they make it harder to hire new ones. This ties into the psychology of loss aversion [0]: people pay much more attention to what they have and could lose, than what they don't have but could gain. One could argue that outlawing noncompetes is important to protect society from the effects of this almost instinctive irrationality in human psychology.

[0] https://en.wikipedia.org/wiki/Loss_aversion