Maybe PlusBus tickets aren't as commonly used as we think. Advertising seems to be patchy, and when booking online it's easy to miss the bit about including PlusBus in your ticket, so you have to know in advance what it is, and that it's available for the place in question. I've never tried to buy one using a ticket machine, so don't know how it's managed there.
I've certainly never known ticket clerks suggest it at point of purchase.