That was pretty much the theme of things back in 2014 when I got into the Technical Preview.
Linux is an open source OS and all the different distros are by different programmers but using just a few base versions for their iteration. Example would be the Linux Mint I run which is built on Debian as is Ubuntu, Kubuntu, Lubuntu, etc. Other distros are built on RPM which I haven't worked with yet. There must be nearly 200 different versions of Linux available, some are good but it's nice to be able to use a bootable LiveDVD [and one of the few LiveCD] to test different distros to find what one likes. Most versions are free but all do take donations. Some versions appear to be aimed more at Servers and business users.
www.distrowatch.org
I don't think Microsoft will fully release the source code for people to build their own version of Windows, one has tried with its Lindows that didn't work out. Going back further there was OS2 that's no longer heard of, same for GeoWorks.
To add to the issues of the Operating Systems is the need for keeping up with all the hardware technical progress. Part of that can be attributed to the hardcore gamers wanting/needing the latest greatest fastest machines, it's a cycle of better hardware seen by programmers able to do more with their software which allows designers of hardware taking advantage of it then the software can be improved, etc. It's like a wheel, never ending.