I would assume that those pins aren't as cheap to get as a new zerk. Plus it is an easy item to get the wrong one of when ordering a new part. If I broke off a zerk, I would go to the next size up, drill out the old one and install the new one. They make them in screw in and press fit, so you have a choice. Replacing a zerk is a lot easier than ordering a new pin, removing the old one, taking the new one back cause it is wrong, ordering another new one and waiting a week for it to get here, hoping it is the right one. Last time I did a pin replacement I finally found a machine shop to make me a new pin and got a refund on my new part. Because it was the same size as standard size stock, the manufactured pin was less than the dealer's pin.
Hopefully you have better luck than I did trying to order one!
David from jax