Oh, but the Trek world has plenty of mental illness. They call it crime and devote entire planets to curing it!
In "Realm of Fear", we get a ballpark figure:
LaForge: "What about the millions of people who transport every day without a problem?"
"Millions" might sound like a low figure for routine civilian commuting if the Federation population is counted in trillions, as per DS9 casualty estimates for the Dominion War. Perhaps only Starfleet personnel use transporters, and they account exclusively for these millions?
Then again, how many people would need to commute in the future? Apparently, the preferred means of motion in most places on Earth is walking, or horse carriages, with only a select few powered vehicles ever seen, and mainly in those big skyscraper concentrations whose prominence and prevalence on Earth has never really been clarified.
Even the most recent vision in Trek, and thus the one with the lowest level of technology in the somewhat annoying pattern of constant dumbing down of the future, features types of habitation other than conventional cities. Iowa appears to sport giant structures that might be industrial plants but were apparently intended to be arcologies. Conventional transport would not feature prominently in those - but teleporting might not be favored, either.