Security. In most cases, a virtual machine has to be an isolated environment, without no access to real hardware.
What would happen if you VM throw an illegal call to the video subsystem or accesses some ports wildly? In an isolated VM, the virtual hardware would fail and the VM could hang or worse; but the host would survive. If the VM is not isolated, it could hang the entire REAL machine. In an environment running several VMs in a server that would be surely a problem; at home it would trash your data.
In "real" VMs (Virtualbox, VMWare), most hardware is virtualized as well as network connections (although a VM surely will have access to the network) and disk (or disks are independent from host disks). Note that DOSBox is more like an emulator: although it virtualizes network, graphics and sound; it "shares" disks with the host. A virus could escape the "virtual sandbox" through disk, but it won't get very far (a DOS virus running in Windows 7?).
I have traveled across the universe and through the years to find Her.
Sometimes going all the way is just a start...
I'm selling some stuff!
