For the same reason automobiles need expensive specialists? Operating them is a no-brainer; keeping them running is hard work for a team of professionals.
We never met anybody in Star Trek who would have been at a loss to operate a transporter. Heck, even the near-caveman Captain Christopher almost got away with it, and Kirk certainly seemed to think his prisoner would be fully capable of using a transporter for his getaway. And that
is with military hardware; civilian models would no doubt be much more user-friendly.