I'm afraid that's a question for DropBox. I don't use DropBox myself. I had heard stories of various problems with it over the past few months. I think they didn't have an Apple Silicon version for a long time. Then maybe they were trashing their app and going all web?
And I think now they are adopting Apple's new 3rd party cloud storage API. That API is designed to give 3rd party cloud providers features equivalent to iCloud. But that also comes with limitations specific to iCloud. One of those limitations is restricting the folder to the user's home directory. Previously people had been able to use an external hard drive. Apparently that's no longer possible.
I can see how a small start up with an app in the App Store might want to adopt Apple's new 3rd party cloud storage API. But personally, I don't think that's appropriate for major providers like DropBox. They have the resources to provide enterprise services beyond what Apple can offer in its consumer product. But that's just me.