I agree it would be nice. But Disney is a business. I don't see what Disney would gain.
The annual passes fall under the category of "Things that are mutually beneficial." By encouraging us to buy annual passes, Disney is also encouraging us to spend our money on-site rather than wandering off to off-site attractions, dining, and shopping, even if we're spending 10 or more nights per 12-month period at onsite
DVC resorts (and would thus be likely to look off-site for more variety and to save money).
Sure, some DVC members bought APs anyway. For others (like us), the economics really sarted working out when the discount was introduced. I imagine that AP sales to DVC members went up substantially when the discount was introduced. And I also imagine that many DVC members bought add-on contracts to be able to take advantage of the APs. That's good business for Disney.
On the other hand, DVC members who don't stay long enough to benefit from APs are probably going to buy regular park passes anyway and to focus on on-site activities anyway. There's probably no business reason for Disney to provide discounts on regular passes. Magic Your Way passes provide great savings as the number of days increase, and DVC members are in a good position to make the optional, extra-cost "never expire" feature work for them because they know they'll be back.