Using the internet it's possible to verify a doctor's job(assuming access to medical charts and lab results).

I think the thing that separates licensed professions from unlicensed ones , is usually issues of safety,i.e. when the price of an error is just too expensive.

So truck drivers(which is a pretty easy to verify job - are goods delivered ? ) are licensed, and have a conduct code("sleep every X hours" , speed limits ) , and computer maintenance technicians aren't, even thought it's more complex to verify their job(are updates installed ? is the computer secure ? ).