Sure, but that does not make it a slow teleport. And silt striders might be safe from ground animals, but let's not forget morrowinds most annoying enemy nr. 1. And I don't remember traveling together with others, I don't remember other people other than the actual traveling agent. This also insinuates that the enemy actually is smart enough to know when to not attack, and I've had cliff racers follow me into the safety of a city, and get absolutely slaughtered by the guards.
Game mechanics, purely and simply. In books (think A Dance in Fire), transports are often full. And really, it's not too difficult a scenario to imagine; a full transport is more cost-efficient than transporting 1 person. A transport that's defenseless along the roads is bad business. Those are pretty much universally-applicable statements. And derived from those statements, 1) transportation in the world will, more often than not, involve other people, and 2) transportation will hire mercenaries to protect themselves.
The actions of enemies, whether they be NPC or creature, are the fault of AI, and it seems rather pointless to talk about AI when we're debating believability through a separate mechanic. "I find transports not getting attacked more believable because bandits are smart enough to pick their targets" sounds a lot more reasonable than "I find transports not getting attacked less believable because the limitations of a game mechanic, specifically the AI, means they will be hostile to anything on sight."
Would it be nice if our bandits had a bit more intelligence? Sure. But mentioning AI in a matter of immersion is like telling the Emperor you don't think the Oblivion crisis is much of a threat, seeing as you have the power to infinitely delay it.
There are guards on the roads, who's to say you don't travel along side one of them as you go across the map? If we imagine you use your weapons, why not your spells? Who's to say you don't use every ability you have to avoid conflict. Sure I would have liked to see this, but not seeing it, does not make it a slow teleport, just as not seeing what you assume happens when that screen fades black in morrowind, doesn't make a silt strider a slow teleport.
Come on, now. If you walk through Cyrodiil, even on the roads, even if you use every skill you possess, you're going to get attacked. There's no denying it. Could you walk alongside the patrolling guards? Sure. But how many guards patrol those roads? What are the chances that you'll know their schedules enough to walk with them? And there are some roads that don't get patrolled. The Red Ring Road gets a lot of activity, but segments of the road to Chorrol, Bruma, Bravil, Leyawiin, or Anvil don't.
This also totally ignores that seeing it as a slow teleport stretches believability to a much farther extent. You don't lose magicka, you don't even have a spell, you will never find a scroll with the ability, not in oblivion at least. You do not teleport.
It is not a "slow teleport" as referring to a spell. It is a slow teleport in the sense that your character is removed from the world and its consequences, while time is fast forwarded, and then your character arrives at his destination unscathed, no matter what. Does Morrowind's transportation system do this as well? Yes, as has been noted. However, 1) Transportation in Morrowind is limited. Spells and Mages Guild transport are truly instantaneous and don't fall under this caveat. Silt striders and boats only go to specific locations. And 2) Transportation a la Morrowind makes more sense to be uninterrupted than simulated walking. When you pay to get somewhere, you're not just paying to get from point A to point B. You're paying to get there safely. And while that doesn't mean you're completely guaranteed no plausible problems, compare it to what Orzorn posted up a ways:
I took a taxi from one side of Los Angeles to the other and wasn't attacked.
I walked from one side of Los Angeles to the other side and wasn't attacked.
Which is more believable?
Despite the fact that it is plausible that both could be attacked, it is exponentially more plausible that you'll be attacked on foot as compared to a limited transport. Hence limited transport's better believability over simulated walking.