I can only guess, but I think the 'pulseOut' is trivial to implement with reasonable accuracy, so they didn't implement it. It's a little bit tougher to measure pulses and remain accurate (and also offer a waiting timeout), so the library implements it for you.
void pulseOut(int pin, int us)
{
digitalWrite(pin, HIGH);
us = max(us - 20, 1);
delayMicroseconds(us);
digitalWrite(pin, LOW);
}
I'm just guessing as to the time bias (20 microseconds), and you might want to go to direct port access to allow for a slightly tighter timing. But that's the gist of it.