Look at it this way, Jomoco: If Bob needs his electrical panel changed, or the knob and tube wiring upgraded, he knows he has got to call in a licensed electrician, right? (He can opt to do the work himself, or get his brother's cousin to do the work) The public knows that electrical work is handled by electricians, plumbing by plumbers, and tree work by any hack they can find who owns a pickup truck, a saw, and a ladder. Who will top or butcher their tree for a reasonable fee.
The big bad government making arboriculture a skilled trade would give it brand recognition, whereas most people likely don't have a clue what an arborist is. Anybody can call themselves an arborist. Can't pull that stunt with other skilled trades (except mebbe carpentry, lol)