Focus on a niche, create a funnel, be very good at that niche, scale horizontally.

EDIT: Picked attorney because you have to be licensed to practice law, creating a barrier to the equivalent of code bootcamps. My apologies if the model around the idea wasn't clear, I will do better!

Why is this seemingly general rule so specific to law (being an attorney)?

The license is the key, as a barrier to entry. Medicine is very high and takes a lot of time but pretty impenetrable. Law is cheaper for the time and money. What else would have this property?