said by cheeper :the lesser kit lenses are cheaper, allowing lower price point for the whole kit.
the lesser kit lenses are faster (bigger aperture), thus brighter in the viewfinder and a bit better in autofocus.
Wrong.
The kit lenses are usually the cheapest junk they can throw on the camera. They are also usually the worst performing of them all.
The reason most people don't get a 18-200 style lens is because it's difficult (Read EXPENSIVE) to manufacture, while still maintaining acceptable quality, and usability (Light intake).
Has nothing to do with brightness in viewfinder, and certainly not focus as kit lenses normally have only passable focusing ability compared to high end models.
A GOOD 18-200 lens is going to be seriously (like 10X the price) more expensive than a standard kit lens. So no camera maker is going to slap a 1000$ lens on every camera, when you can easily put a 100$ lens on.
But back to the OP's post, yes you CAN buy a wide range lens like that (There are even some crazier ones out there) but you're going to pay a serious arm and a leg, it's going to lack light performance, unless you pay another arm and leg, and at that point, you're not going to want it.
Unfortunately it's nearly impossible to get a "have your cake and eat it too" lens. For the simple fact that the wider range you give it, the (exponentially) harder it becomes to manufacture while maintaining quality, thus the (exponentially) more expensive it becomes.