The problem
You find a plumber by asking around
A neighbour knows someone, who knows someone. When it works, it is perfect. When it does not — the job is botched, the price doubles at the end, the person never comes back — there is no recourse, and no way to warn the next customer.
And there is the opposite problem, which is talked about less: a good professional with no network takes years to become known. Their work is not enough; they need to be recommended.