Part of it is practical, it is easier to virtualize other OS's on OSX than the other way around, but also don't underestimate the 'status symbol' of 'look, my employer spends money on me'.
It's also the best platform for mobile app development as it natively supports both iOS and Android dev.
How is it easier to run VirtualBox on OSX than Windows?
It's easier to run a Windows guest vm on an OSX host, than to run an OSX host on a Windows or linux host, because of licensing, so if you need a mix of platforms, having OSX be the host is the path of least resistance.
Symlinks in shared folders
This is possible on a Windows host in a hackish sort of way, though I imagine it's more seamless on a OSX host, being Unix.