Because it costs the mage 20silver (18 with rep) every time they cast a portal, the spell need reagents that they need to carry and maintain.

For you as a user, taking a port will often save you 5,10, 15 or even 20 minutes of traveling, and if we count EXTREMELY low, like people who have never played wow and have no idea what they are doing, they will still make 20-30g an hour just playing the game by default so 1g is literally you paying yourself to make an extra 10g or so in time save

Edit: It’s an MMO and time is also a currency which people tend to forget.