I'm having a little trouble teasing out the actual question/premise of the original post unless it is, as has suggested, just another way to take a dig at what the OP considers inferior bikes.
I will say that not everyone is in a position to buy a custom bike with their exact, preferred component set. Not everyone needs a custom frame. And many people learn by actually riding their bikes on tours or around town and then learning what changes they'd like to make. Why don't they just buy a custom bike with their exact choice of components? That seems like a pretty big risk (unless you have money to burn) if you don't have the experience to know what you want. Especially when it comes to a custom frame.
Personally, I feel like I'm a long ways away from getting a custom frame, partly because of the added expense, but partly because there's such a wide variety of frames available, that I'd have to want something really specific to not be able to find a frame to suit my needs.
I'd also agree with [MENTION=104720]alan s[/MENTION] Alan S on the issue that choosing your own components does not necessarily mean spending more. If you know what you want, if you're patient, you can find the parts you want at the prices you want. I bought my last two bikes as frames and selected all the components myself. In both cases, I spent some time looking for deals, and built the initial bike out of parts on hand, replacing parts as I obtained the pieces I wanted. Not only was it cheaper than presenting a parts list to a builder, but it spread the cost out, and it made me much more aware of how everything went together. If I had gotten a quote on my "perfect" bike from the shop, I'd still be saving my pennies to get there. Instead I'm riding my bike.
Why do touring bikes get customized? It seems natural to me. I'm not one for road cycling as a sport, but as an outside observer, it seems like the goal is to go fast and keep things light. That's pretty clear cut, and while I am sure there are plenty of places where personal preference comes into play, with a common goal, it seems like it's going to be easier to make bikes that are ready to be ridden.
Meanwhile, what are the goals of a touring cyclist? Some people feel like they've made good progress if you've moved yourself 30 miles down the road by the end of the day. Some people feel like they haven't even gone anywhere if they don't at least do 100. Most people are in between. Some people carry a change of clothes and a credit card for the hotels. Some people carry everything they need to eat, sleep, and keep rolling indefinitely. Some people stay on pavement. Some people would stay off roads entirely if they could. And almost everyone has one idea about their riding preferences before they start, and different ideas once they've done it for a while.
How can there be a standard bike to cover those needs? And how can you know how your needs differ from the available bikes before you spend some time on them? I'd find it more surprising if we could all agree on a single bike and set of components. Nothing makes more sense to me than someone who chooses to spend a good amount of their day traveling by bike having opinions on how that bike should be put together.