They sell the Pi Zero (W) at such a low price to promote the platform for educational and non-commercial purposes. However, the margins are so low at that price that they cannot afford to sell in bulk. They apparently do offer bulk buys (>500 [1]) at higher prices.
The few options to buy multiple are to buy kits (which basically pass that bulk pricing in the form of overpriced accessories), or at a higher price (e.g. I see $22 on Amazon), or be very patient and keep ordering.
[0] https://www.raspberrypi.org/forums/viewtopic.php?t=198705#p1... [1] https://www.raspberrypi.org/forums/viewtopic.php?t=201916#p1...