OK, two example points using the Forgotten Realms as an example:
1) Remember the invasion of Cormyr in the "Death of the Dragon" novels? What if there were teleport circles between Cormyr and Waterdeep for 'easy travel'? Khelben or Maskar Wands would have shut the darned things down so fast it wouldn't have been funny! Otherwise, there would have been armies of Goblins roaming downtown Waterdeep, pillaging and looting like crazy! Generally, good commerce or no, rulers do not like teleport circles in their major cities.
2) Faerun is ALREADY littered with magic gates! Wizards have been setting up the things all over the place for millenia! Listen to Elminster for a while and you will find gates, sometimes totally by accident, set up by a wizard 500 years ago! Sometimes, you wande into them, sometimes, you have to walk between two petrified tree stumps and speak a special word, etc. But they are all over the place!
Plausibly, a teleport circle would be a major undertaking for a wizard, and yes, there could conceivably be several linking major cities of an empire together. However, even after, say, two thousand years, there would only have been 5 wizards or less who would be both powerufl enough and willing to create such a thing. Unless, of course, you play in the Forgotten Realms.
More plausibly, you would have middling level wizards who charged exorbitant fees to travel special teleport routes to get you where you needed to go, much like in modern times businessmen charter private planes to take them to special locations.