If I want someone to do work, I want them to be licensed/certified. If they are flagrantly unsafe, I want a licensing board or similar to be able to strip that person of their ability to practice that profession. This raises public perception of the profession as a while, avoids a market for lemons, and gives some baseline.
There are too many decisions in life to be able to spend an hour (or more) researching every option. Credentials allow a framework of trust - I don't have to decide if I trust every single engineer; if they have passed their PE exam and not had their certification taken away that is a starting point.