"In which case, why do we need an OS in each virtual machine?"
We never did...and we didn't need no steenkin' VMware either till Windows came along and started pretending to be a server OS.
Unfortunately the IT industry, particularly management's obsession with virtualisation came about with the ever increasing presence of Windows in the server space over the last decade. And it's been buzzword driven to ridiculous extremes and the unix world has had to play along with it. It's a solution to a problem that never was, on Unix. Like several others smarter than I have commented above, every imaginable "resource" can be isolated and controls set between disparate applications and it's processes under one instance of the OS. Virtualisation with multiple OS instances on unix is redundant in most cases and serve no purpose other than to sell more software and support licenses. I suppose having more OSs to manage also helps Unix/Linux admins appear busier to management with their limited windows only worldview.