Because it's Unix

Threads are out:

Don’t ever let anyone tell you that fork(2) is bad. Thirty years from now, there will still be a fork(2) and a pipe(2) and a exec(2) and smart people will still be using them to solve hard problems reliably and predictably, just like they were thirty years ago.

Ryan Tomayko

Watch the cascade from Ruby is Unix to Python is Unix to Perl is Unix.