This is not missing. For example qubesOS has very strong process isolation, spinning up seperate read-only VMs that can (optionally) be completely disposable, so can safely be used to run actively hostile code if required.
VMs are substantially heavier than process isolation. Isolated processes can share certain resources, VMs often boot a separate instance of absolutely everything they depend on (down to the kernel or even emulated hardware).
That was an example to show it's not something that does not exist in the linux world. There are less extreme examples through the whole spectrum of isolation, from firejail at one side (which uses capabilities) through a couple of container-based approaches all the way to qubesOS, which is the most robust from a security standpoint I would think and is also the example I chose because I'm most personally familiar with it.
https://www.qubes-os.org/