There are both practical and market reasons.

Historically, shift-only lenses have existed, but photographers generally preferred having both movements available in one lens. Tilt-shift lenses are already a simplified substitute for a view camera or bellows system, where movements can be used independently and in more directions. If you split tilt and shift into separate lenses, each lens becomes even more limited.

There’s also a practical benefit to having both on one lens: on many tilt-shift designs, tilting the lens can also move the image relative to the camera’s centerline, causing an apparent framing shift. A built-in shift mechanism lets you compensate for that without moving the camera.

So while separate movements are possible, combining them makes the lens more versatile and helps correct side effects of tilt. Cost is not necessarily driven by the presence of both features alone; these lenses are niche products made in relatively low volumes, which also keeps prices high.

If you want fully independent movements, a view camera or bellows-style setup is the more complete solution.