The problem is that while the technical challenges of making a more user friendly CNC are trivial, the expense is not.
They need strong metal parts machined to great precision, gearing, high torque/high RPM motors etc. And a self-indexing CNC that does even more positioning etc. would just cost more, you've just added more axes, and degrees of freedom to the device. More parts, more gearing, more motors etc. And there's space/size/power considerations that don't go away with CNC's either. Block of steel X, cutting of size Y, using bit Z will pretty much always need Watts/Amps of XYZ.
With a 3D printer, you only need 3 axes of precision, and as long as the part is smaller than the workspace, repeatability and indexing is only critical within the making of the part. If you're off by even an inch or more in terms of being centered in the work table, or powder bed depending on what kind of printer it is, it's no big deal. Not to mention the ability to make voids that no CNC device could ever produce.