why do people?

You have a valid point and many people do include the skp. files.  I can also say that I have intentionally left out the design file for certain things.  My reasoning was simply to provide a fool proof printable file.  I have done this for particular files that for some reason or another were near impossible to make manifold.  Usually for those types of files it is easier to import them into Sketch-up and then make the changes.  I have actually had  designs that were difficult enough that I can't seem to make a change without messing the whole thing up.  When this happens I have run it through netfabb and then imported it back in to Sketch-up and then proceeded with the changes.  Even though the clean up of the imported design is a pain it really doesn't take that long.  Of course I am no designer and I am certain there are better ways to get the job done.
Most people are happy to share their design files if you just ask.  I have never had someone say no so I am guessing the real reason is they just don't take the time to upload the files.