Protecting identity credentials is costly:
Having multiple credentials is not only smart from a "not putting your eggs in a single basket" perspective, it's also more efficient. No sense protecting your Starbucks Coffee Card to the same level you protect a passport. Real security won't come from a single, all-powerful identity credential, but from a healthy ecosystem of useful, practical, and effective identifiers.
And let's be clear: Real ID is an "all eggs in one basket" solution. Its standardization means that EVERYBODY will have equipment to swipe it, and fake RealID cards will be worth tens of thousands. The states will have to hire SO MANY new employees to process Real ID cards; do you suppose they might hire a villain or two along the way?
Well actually, Real ID is not truly an "all eggs in one basket" process. When you consider Passports, you really have two nearly identical eggs for each person to keep in their basket. Please tell me, why do we need passports if we have Real ID? Or vice versa?