Because it can't be done anywhere else..
Yes, you can do it in user space, but your performance will suck unless you
ignore security issues or have special hardware (and the special hardware
would essentially have to do 90% of the stuff we do in kernel space now: I'm
talking _really_ special hardware).
It's been talked about (favourite thing in microkernels is to move critical
resources to places they don't belong ;), but I doubt _anybody_ seriously
thinks that it can be done in user space while keeping up with things like
HIPPI and having full memory protection.
Oh, and the big problem isn't throughput: you can essentially get throughput
by having large buffers (modulo the problems with maxing out your memory
bandwidth which in itself is a large problem). The _big_ problem is latency.
In short, forget about networking in user space unless you're talking >10ms
latencies and <10Mbps throughput. 10Mbps ethernet may still be realistic in
user space. Just. But the world is moving to 100Mbps and beyond.
Linus