Except they have, starting with the iPhone 4. Why do you think they named it that? It's not like it had 4G or anything. No, it was the fourth iteration of the iPhone, so it was called the iPhone 4, simple. Now we have the 4S, which doesn't really mean anything. Is it the fifth iteration, as the 3GS was the third iteration? Is it the 4.5th iteration, even though Apple are claiming it's an all-new phone? So if they come out with the iPhone 5 next year, it'll be the sixth iteration of the iPhone, and that wouldn't make much sense.
Where are you confused? The "s" phones show a pattern of an upgrade while maintaining same case design. More time to make more money before investing in molds for next gen phones.
So by your extrapolation, by the time Apple gets to the iPhone 6, they'll have released 8 phones? What kind of logic is that?
The only reason the 3GS was called the 3GS is because it built off the 3G, which was introduced when Apple had a different naming scheme, calling the phone by the type of wireless technology inside it. Then with the iPhone 4, they introduced a new naming scheme, going by the iteration of phone. But now they're sort of doing a hybrid, and it really makes no sense.