While a deck bag and a regular backpack are going to have a lot of similarities, they are very different products. A deck bag is intended to be attached to the front of a kayak and will have a contoured shape to ensure that it is as easy to access as possible.
A regular backpack isn’t going to be able to be attached in the same way and instead will be designed to be able to be worn around the shoulders. There are backpacks though that are going to be good to wear while kayaking for additional storage.