The reason orgs use Sharepoint is they are forced to if they use Microsoft. One drive is sharepoint, teams is sharepoint, sharepoint sites is sharepoint, etc...
I'm sure all those things have better alternatives, but Microsoft shoves them down your throat when you license with them.
You’ve got it backwards. Everything M365 is an amalgamation of Entra, SharePoint, and Exchange.
I'm not saying that wouldn't be better, but it makes sense why an org would be reluctant. Again, not a fan of Sharepoint myself, but from an org's viewpoint, moving to Linux raises more problems than it solves.
To some extent I think Microsoft is largely in the business of building solutions for problems that don't exist.
Most orgs are probably perfectly fine with a document management system + desktop word application and then a commercial NAS for bulk storage / backups.