When using a tilt shift lens I find myself using the two features of tilting and shifting for separate photos. If I want to control the depth of field such as in a portrait or macro shot I will tilt the lens. If I want to control the perspective such as in architecture I will shift the lens. That brought me to thinking, they both seem like very complex movements of the lens, so why not separate them into their own dedicated pieces. This would seemingly save on costs and simplify the design significantly. I might be missing a key feature of combining the two features together, but I have not encountered that so the question still perplexes me.
Well, to begin with, a tilt-shift lens is a rather limited substitute for a proper bellows setup with free movement of the front lens. Offering only shift or only tilt makes that even worse. That said, shift-only SLR lenses have been made, such as the 35mm Nikkor PC "Perspective Control" from the early sixties. Canon answered with a tilt-shift lens in 1973, says Wikipedia, and after that the marketplace hath spoken - yes, we'd like both tilt and shift in one lens, please. Anyway, a T/S lens is in principle just a medium-format lens on a purely mechanical tilt/shift mounting, it's not exactly rocket science to design and manufacture this.
The high price has more to do with very limited demand and correspondingly small production volumes (plus that the Canon versions at least are absurdly well-built, similar in feel to early EOS L lenses before they started to use plastic), dividing it up into separate tilt and shift versions could only make this worse.
All the SLR tilt-shift lenses I've used have had a tilt action that shifts the tilted lens elements relative to the centerline of the untilted lens, resulting in a noticeable and significant shift of viewpoint when tilted. Including a shift mechanism (that can be rotated to act parallel to the tilt mechanism) provides a simple way to compensate for that shift when positioning the camera to accommodate it is not feasible.
Sounds like you really want a view camera. They are great for controlling dept of field during product shots. or for hard core macro shots. I'm a bit confused how you'd use it for portraits, most of the time, you want the eyes in focus. View cameras are a PITA to use, but they can tilt separately from shifting, tilt in two axis, etc.
Used view cameras are cheap. Some bloggers, such as Alex Koloskov, use a FF digital camera as the back. There are, of course, medium format digital backs, but they are wicked expensive.