Some corporate travelers are working with travel policies that will reimburse any economy ticket, but no premium ticket. So even though Y is more expensive than Z, they buy Y.
And, if you're on such a policy, why bother buying a cheap or W fare? It's not your money... and buying a lot of Y fares is a good way to qualify for GS, in addition to putting you at the top of the upgrade list. (Of course, this presumes the traveler doesn't care about their employer's bottom line...)