Your status trumps (or, probably more accurately, supplements) the inherent features of your ticket.

If you already have SkyPriority via your status, what would be the point to require you to buy a ticket that also has SkyPriority as one of its features in order to actually get SkyPriority? (That would be like saying: Good news - your status allows you to check one bag for free! However, to actually check that one bag for free, you have to have a ticket that already includes one free checked bag !)

If you, as a FB Gold/Platinum/Ulti or other Skyteam Elite Plus member, have to buy a ticket that already comes with SkyPriority in order to actually obtain SkyPriority benefits, then it would actually mean that your status does not get you SkyPriority.

People don't seem to struggle with the concept that their status gives them "add ons" they can pile on top of any ticket - an extra checked bag, and lounge access, for example - even on tickets that specifically do not already include these features. I don't see why people struggle to view SkyPriority as a similar "add on" that their status allows them to pile on top of any ticket - whether that ticket already has some/all of those features included, or whether that ticket has absolutely none of these features (as in the case of Economy Light tickets)