Within the realm of Cartesian machines (eg having orthogonal X, Y, Z axis) consider the sort of design you do see: the Ultimaker-like X-Y stage over a build platform which can drop.
The question is then, why drop the build rather than raise the mechanism?
Likely the reason is mechanical simplicity. The X-Y gantry is the part that moves around quickly and may vibrate. It's also the part that is rather complex and has lots of electrical cables. Making that fixed in the housing is generally a simplicity. In contrast, the built object in a typical consumer printer has lower mass, and the build platform only moves slowly so variation in dynamics from its increasing mass isn't really an issue (increasing build mass typically isn't even really taken into account in designs where the build rides on a rapidly moving Y axis either).
You might ask, what would a machine like this look like if instead of dropping the build you raised the mechanism?
It turns out such a thing does exist, in the form of a clever hack where you run a linear rail up the wall of your hackerspace, mount your entire Ultimaker-style printer on it (less build platform and bottom panel), and tap out the Z control signals such that the printer climbs itself up the wall leaving some towering sort of build on a fixed platform below. Conceivably if you wanted to start there you could also saw off all but the top third or so of the cabinet to leave the X-Y gantry with some rigid frame. But it's a bigger, more expensive product that doesn't work simply by lifting it out out of the shipping box. Apart from very unusual or "proof of concept" builds, it doesn't make a whole lot of sense.
The thing about the 3d printer realm is that you're free to try anything you like. Some ideas work. Some ideas don't. Some that go a little bit against orthodox mechanical design as taught in Mech E. departments turn out to work a bit better than they should fairly be expected to, and make it into products. But generally what is on the market is what has proven to present a good balance between cost and utility.