You are of course completely right. Almost everybody who works on Sage does so as part of some research mathematics project, often in their spare time, and they just don't have time do the massive amount of work to bring the documentation to the same level of user friendliness as Mathematica. I hope SageMath, Inc. will be able to makes strides to improve documentation for open source math software. Also, this by Greg Bard is a bright spot: http://www.gregorybard.com/Sage.html