It needs device drivers for modern x86 hardware. And filesystems, and a TCP stack. All of that code can be done in "SeL4 userland", but yeah - I see your point.
Are there any projects like that going on? It feels like an obvious thing.
A lot of deployments essentially virtualize Linux or run portions of NetBSD (e.g. via their "rump" kernel mechanism) to achieve driver support, file systems, etc. That's not really a general-purpose solution, though.
There is work within major consumer product companies building such things (either with sel4, or things based on sel4's ideas), and there's Genode on seL4.
