Because Linux not an OS. The flagship OSS OS is Ubuntu, and it's mostly pretty stable. But OSS inherently implies the ability to make your own OS that's different from someone else's OS, so a bunch of people did just that.
(OK, I have some experience with vendors where their latest month-old release has an distro support release where the most up-to-date option is still 6 months past EOL, and I have managed to hack something together which will get them to work on the newer release, but it's extremely painful and very much not what either the distros or the software vendors want to support)