Probably because most Linux users aren’t looking to share (or even use) office documents. Linux collaboration happens on wikis, message boards, and Git.

There's Liferay:

https://www.liferay.com/resources/l/content-management-syste...

https://hub.docker.com/r/liferay/portal

I haven't played with it in about 5 years but it was substantially less polished than a well-run Sharepoint 2013 instance.