Here is why FOSS docs are so nice to you, but proprietary ones are not: audience analysis.
Exactly. The OP gives himself away when he describes his computer use as a "home hobby".
As a pro, I much prefer the nice, succinct, "straight-to-the-point" man pages that you find with open source stuff than the tediously long novels that come along with commercial software.
I'm sure hobbyists would prefer a "for dummies" version, but I just don't have the time to read 30 pages of rambling bullshit just to figure out what the "-x" command line option does.
Personally, I think OSS documentation is, for the most part, exactly what it should be.
Alternate answer:
Docs for commercial software are written by professional "technical writers". Many of them paid by the hour. ALL of them incapable of understanding the details of what they're writing about. Their job is to describe the software to the least common denominator. Many times, the person writing the documentation IS the least common denominator, and couldn't make good use of it to save their life. What they do understand, is that the more words they put in the doc, the bigger their paycheck. So you end up with 750 pages of bullshit that doesn't actually explain how the program works.
OP should fire up a Linux system and type "man rsync" and "man bash", read them top to bottom, and then ask himself why his own inability to comprehend that excellent documentation leaves him thinking that OSS docs aren't up to par.