Dokan for Windows (last updated 2011) and MacFUSE for OS X do exist, but they're not battle-tested, to say the least. It wouldn't be wise for Dropbox to stake its reputation on such unreliable software.
MacFuse worked well when it was first released, but it has not been maintained in years and has no maintainers and does not really work with any recent OSX versions at the moment.
It looks like there might be something called OSXFuse which might be more recent and maintained, but I haven't looked at it, it also might not be.
Commercial FUSE/Dokan analog for Windows costs around 15K for an all-inclusive package. I'm sure Dropbox can afford it if they are interested in pursuing this option.
