Why is Teams Cross-Company Auth So Terrible?

Every project I'm on in the federal aviation space has a Microsoft Teams share for collaborative document development and sharing.  These projects typically involve people from at least three different organizations, distributed across locations and timezones, so real-time collaborative editing is a key enabler.

Microsoft's collaborative editing functions work really well, especially given the concurrent access and revision tracking bits that are happening behind the scenes.  It's pretty flawless.

However... on most of these projects there are least one or two people who simply cannot get access to the project share, no matter how much effort their repsective IT teams devote to it.  I mean, everyone wants to use it and everyone is paying for it.  It's a huge drag on the projects to be emailing copies of things around for review and edit.  It re-introduces all of the problems that online collab solves.

Microsoft has basically infinite money to throw at making auth smooth and interoperable between organizations, and somehow it remains terrible.  Possibly this is just a matter of certain IT teams not knowing which levers to pull, but I have my doubts.