It's always amazing with worlds like this, how a thousand years before armor and technology look exactly the same as they do in the current games that are apparently set far later.
Suspension of disbelief only gets you so far.
That's usually my one gripe with fantasy settings. Some of them (and in my head I'm thinking about the Game of Thrones Universe) have had civilized society for thousands of years, yet still rely on horses, fire, etc. No steam, no electricity, nada.
You would think they'd have more technology by then, if you juxtaposition it to the real world's timeline of events regarding human civilization and inventions.
Well there are still civilizations in certain areas of the world living a stone age lifestyle, completely unaware of modern technology.
Advancement is by chance.
Wouldn't that be for powerful mages though? The kind who I doubt would be very interested in carrying letters around for people? It would be like going to school for 10 years to get your PhD in engineering and then the job you get offered is carrying letters around for people.
No, it was just one of the two fast travel options and was available to any member of the mage's guild (or House Telvanni I think) for a small fee. So while it wasn't easily available to everyone whenever they want, it's clearly common and easy enough to allow many people to engage in regular long-distance travel.
Importantly, remember we're not actually talking about something like a postal service, we're talking about how major technological advances might be able to spread across a fantasy world. A powerful mage might not want to spend their time delivering regular people's letters, but they may well be interested in carrying news of a working steam engine or gun to their fellow mages. The point is that you don't necessarily need widespread global communication like the internet for things to spread, you just need such communication to be possible at all, even if only to relatively few people.