While there are some waterproof day packs on the market, it is not a super common thing to see. That’s generally because waterproof materials are not always the best ones to make weight-supporting backpacks out of. In addition, these kinds of materials are hard to sew things into without ruining the waterproofing, so things like external pockets and straps wouldn’t work as well.
To get around this, most hunters simply utilize a separate waterproof pack cover for bouts of inclement weather. They are small, lightweight, and can fit easily into an external pack pocket for quick access when the weather goes bad.