I think it's the fundamentally non-scalable nature of the work. You see it in other professions, e.g. venture capital and estate management. If it's possible t

I think it's the fundamentally non-scalable nature of the work. You see it in other professions, e.g. venture capital and estate management. If it's possible to get more work than you have time to perform, then you can be selective about what work you take. Consequently, referrals from a customer you've previously done business with are less risky / hassle than internet strangers. The difference between me writing code for 1 person or 1,000,000 people are relatively negligible. The difference between a lawyer working 1 criminal case and 1,000,000 is much larger. Work product doesn't scale as evenly.