It's probably safe to assume that a scout upgrade is something that the developers considered at some stage, what with the existence of the explorer unit in Civ4 and all. It's unlikely something they just didn't think about. So there must be a decent reason why they opted not to include another unit.
I would imagine that it would have to do with the lack of any feasible upgrade. Given high maintenance costs, you can't just have a unit hanging around doing nothing while it waits for an upgrade late in the tech tree; there must be a series of upgrades that it can work its way through. And each of these units has to be of some use. So what would come between scouting and archaeology, if that's what it were to upgrade to eventually? There's really not much use for some extra sort of scout after that first unit; there's no scouting hole to fill, as such.
One possibility would be to have some sort of colonial function, but that's a fairly in depth feature, and wouldn't be something the developers would just extrapolate from "we need a scout upgrade".
I would imagine that it would have to do with the lack of any feasible upgrade. Given high maintenance costs, you can't just have a unit hanging around doing nothing while it waits for an upgrade late in the tech tree; there must be a series of upgrades that it can work its way through. And each of these units has to be of some use. So what would come between scouting and archaeology, if that's what it were to upgrade to eventually? There's really not much use for some extra sort of scout after that first unit; there's no scouting hole to fill, as such.
One possibility would be to have some sort of colonial function, but that's a fairly in depth feature, and wouldn't be something the developers would just extrapolate from "we need a scout upgrade".