I can't fathom why, but many people DO pay rack rate. The current model allows for a (perfectly legal) system of price discrimination. Disney can collect $100 a night from those willing to pay it and/or too lazy and/or uninformed to look for discounts. They also collect $70 from the guest who knows about the discounts, seeks them out, and wouldn't stay on property otherwise.
If they JUST charged $70 for everyone, they'd miss out on the incremental $30 that the first guest is willing to pay. Likewise, if they JUST charged $100 for everyone, they'd lose the $70 guest entirely. It's no different than airline tickets, cruise fares, rental cars, etc. Once announced, Disney won't adjust rack rates up in a given year. Since it essentially functions as a self-imposed price ceiling, it makes sense to set it much higher than projected market rates so that if guest demand and the economic climate slide market rates above projections, Disney has the ability to adjust up. From an optics perspective, it's better to offer fewer discounts than to raise prices. In other words, Disney might expect All Star to rent at $70 per night. They set rack rates to $100 and plan on a 30% room-only offer. If the market indicates that guests are willing to pay $80, Disney simply tweaks the promotions schedule and releases a 20% offer instead of the 30%. This all happens behind the scenes, which would not be the case if they had ANNOUNCED $70 as the rack rate and then suddenly raised it to $80.