Indeed. But it's also very easy, and rather more profitable, to station burly guards in front of it and change an extortionate toll.
So, as the navigators of the Age of Discovery did, people will seek out alternative routes, even if they are dangerous.
And the only place you are likely to find a stable portal that goes where you want to is Sigil. And who wants to go there? Tolls aside, these are the people who call the rest of the multiverse "clueless" after all! Better to give the place a wide berth.
Even Sigil isn't much hope for that kind of trade. If it was, it would be swamped with traders from every prime world seeking profit, but it's not. Why? Because...
"But thing is, these portals do not open when a cutter simply passes by (with the rare exception), for
each requires a special key to open. What is this key, you may ask? Well,
it can be anything, really: a literal key made of a specific material, a thought or simple gesture, or a complex ritual like playing the flute and skipping back and forth three times whilst doing so. The Lady of Pain herself governs these portals, and it is on
her whim that they disappear or reappear in new and unexpected places, more often that not causing trouble for a berk who just happened to be at the wrong place at the wrong time."
There are very few stable portals and even if you manage to find a portal to where you want to go, good luck figuring out what out of everything in the multiverse will trigger it, AND before it up and vanishes on you.
And...
"The only way in or out of Sigil was via its innumerable
portals. Any bounded opening (a doorway, an arch, a barrel hoop, a picture frame) could possibly be a
portal to another plane, or to another point in Sigil itself.
In addition, portals could be permanent or temporary, linking to fixed or shifting locations. Thus, the city touched all planes at once, yet ultimately belonged to none. Those characteristics warranted Sigil the other names it was known for: it was called
"the City of Doors" for the sheer number of portals, but was also called "the Cage" for the difficulty to enter or exit the city"