It's a microkernel: it is intentionally minimal. Microkernel operating system design is from the 1980s is different from the monolithic, bulky and older operati

It's a microkernel: it is intentionally minimal. Microkernel operating system design is from the 1980s is different from the monolithic, bulky and older operating systems designs you are used to (famously including Linux, see the Tanenbaum-Torvalds debate). It has all the features that it would likely ever need already. Instead of having all the services required for an operating system in the kernel, the kernel just implements the bare minimum including fast IPC. Things that traditionally are implemented in the kernel (filesystems, device drivers, other modules, etc) are all implemented as services living in userland and are assigned the minimum necessary additional permissions for them to perform their task. You can build a *nix like system on top of seL4. You can even run Linux itself on top of seL4, with L4 acting as a hypervisor. An operating system doesn't consist of just the kernel.