Because it defeats the convenience of the Magic Band/room charging system. Because if you've been saving for a while using gift cards it is a pain to carry them around. Because it is an annoyance at the cash register to use more than one card or keep track of the remaining balances on cards as you go. Because if a gift card is lost or stolen anyone it can't be recovered and anyone can use it, unlike the PIN protection the MB offers.
We pay for all our Disney trips in gift cards we purchase through a school fundraiser. It is much easier to stop by the front desk every few days and apply the cards than to carry them around (usually in $100 increments - a week's spending can be quite a pile of cards!) to spend at each point-of-sale where we make a purchase. I don't care if Disney changes the established charging limits, but it would be nice to have some sort of notice before the CC is charged or at the very least new "official" limits made known so those of us who do use gift cards can adjust to the new policies. Or better yet, they could start accepting gift cards in advance like they do with cash and only charge the CC on file if you go over the deposited amount.