Why would they? None of the others are oversubscribed so it is guaranteed that the physical machine has the resources to provide everything that the virtual machine wants. Except network, but AWS doesn't guarantee anything about network speed.

In the case of hardware failure, say a malfunction or an error detected by ECC RAM, most people would prefer the machine to be turned off and they can restart it, rather than continue in a potentially corrupted state. As all the storage is network attached, it can immediately be restarted on another physical machine.