This isolation comes at the cost of dramatically decreased performance.

http://dtrace.org/blogs/brendan/2013/01/11/virtualization-pe...

Hardware virtualization is best viewed as a hack for running multiple operating system kernels at the same time, where each kernel is designed to have a machine to itself. In any sanely designed system, this shouldn't be necessary; multiple processes under a single shared kernel should be good enough.

While that is a nice ideal, reality tends to start creeping in eventually, and a hack or two become necessary so work can get done.