I think 3D modeling software is a big reason 3D printing hasn't been the home revolution.
I've been using computer based 2D drawing software since MacDraw in the 1980s and have used it for drafting home improvement projects, woodworking projects and floor plans. I've downloaded Sketch-Up a few times and always found myself baffled quite quickly, even tinkering with generic rectilinear shapes.
And even drawing some boxes or other regular geometric shapes doesn't get you very fair in a world of tapered curves, irregular shapes, etc, let alone the same needing accurate scale and tolerances down to the millimeter.
And it's not that it's impossible, either, but it's got a wicked learning curve over 2D just doing the drawings let alone the phase where you have to consider how you design will actually be output by the thing making it.
Strangely it's almost the blade-and-razor model in reverse. In theory, they should give you the razor handle (the easy to learn 3D design software) for free so that you'll buy the 3D printer and supplies, but I suspect that in terms of cost, the easy to use 3D modeling software is the actual expensive part and the 3D printer should be the cheap part. It's kind of like 2D design software -- an annual contract for Adobe Creative Cloud is almost more expensive than a decent color laser printer.