Sorry, but that's not exactly correct.
They DO price-bridge tickets purchased as part of a resort package.
But (and here's the part that's difficult to explain,) most times that bridging is "invisible" to guests.
The fact is, the actual COST of a park ticket that is part of a resort package is UNKNOWN to (never seen by) guests.
The actual price is only known internally to those CMs in the resort booking offices.
The actual price paid by guests for the tickets in a resort package is LESS than the Disney ticket price list.
So, since there's no way to know what the actual ticket price paid by the guest was, Disney allows price-bridging of the tickets
so that the guest gets the full "Disney price list" value for the tickets when they are upgraded.