In principle, I agree with you. However, in practice, a lot of what drives non-competes is concerns about poaching. I don't know if non-competes are particularly prevalent in service industries but I'm probably most aware of examples from consulting. I'm not going to defend non-competes but I suspect one of the reasons that they exist is that poaching clients and use of inside knowledge from a former employer are difficult to prove if they're not blatant whereas working for a competitor is pretty straightforward.