I think you are confusing selling pot and coke to the local HS/college kids and selling crack on the streets.
The individual selling pot and coke to HS/college kids is selling almost exclusively to friends and friends of friends. He carefully cultivates his contacts and (should) act in a business like manner. He buys from some other low level dealer and makes that kind of profit because they are the only game in town.
Selling on the street is way different. It's gang work mostly. To make one sale you have 4 to 6 people involved. 1) The person who takes the order 2) the runner who takes the order back to the safe house 3) the second runner who takes the drugs from the safe house 4) the money collector 5) the person who makes the final drop off. Each person in that chain needs to get paid along with the rest of the gang.
Then you have the risks. There are two big risks for the college kid: Cops and getting killed when he goes into the city to pick up his supply. The street dealer has many more. 1) Rival Gangs are always looking to cut into the turf, find safe houses to raid. 2) Inter gang politics 3) "Clients" killing over their next fix 4) Getting busted by the cops.
So yea if you are living the suburban dream selling pot and coke will pay the fancy bills while you get your MBA.