Dropbox expects you to pseudo-merge exactly one personal account with exactly one business account. You end up with a mess. It gets even worse when trying to unmerge them. And is not useful should you have more accounts than exactly one of each. That mess will keep coming back to bite you almost every month as Dropbox doesn't do what you'd expect.
Dropbox is lucky that that they do two things their competitors don't: they support Linux and syncing generally works.
I think for Dropbox to operate in a pure filesystem-native-first experience they're probably approaching it from the 1 computer = 1 person standpoint (or one logged in user on the computer at least).
I was around when they introduced the "multi account" support, and it was very easy to see how/why they did it. Instead of updating their clients to support multiple accounts (which they should have done), the client just keeps supporting only one account. Then on their back end they return "merged" content to the client. You can easily see how this can create chaos, because other people can also be updating your personal and business content, and untangling it all is a mess.