Why should lawyers discipline lawyers? Why should cops discipline cops? Why should lobbyists advise legislators? Why should banks help to shape financial regulations? Why should corporate aircraft engineers participate in the aircraft certification process?
Because they are subject matter experts. Who else is better placed and better suited to do it, and who else would charge less for the service?
"Quis custodiet ipsos custodes?" a.k.a. "Who watches the watchmen?" is a 2000 years old quote so it's not a new problem either.
What we want is something that works for the people being served. Subject matter experts are great for input, but just because foxes are experts in chickens, being so intimately familiar with their ins and outs, that doesn't mean that they should run the hen house. We need other stakeholders involved too. The people who depend on those systems need to be represented. The people those institutions serve. The people who will be impacted by them.