Because the taxi is just feet away from where they stand, go through two doors and you are there. It isn't worth it, you have already carried your bags MUCH further. Secondly they are unofficial "agents" for the cab drivers and it was my experience that is the only time the rate was increased beyond what it should be was when my wife involved one of these guys.
I'm not cheap, I always add a good tip for the driver, but the drivers in BC will charge you more if they can. Recently a cab from the water taxi to the Princess cost me $7BZ. On the return the guy wanted $10BZ. Not cool. The rates are not metered like in the states, they can charge whatever you will pay.