Regular backpacks often don’t have as many pockets as these bags do, and they don’t have pockets specifically designed for baby products, such as insulated bottle pockets, a diaper organizer, etc.
Not only that, but baby bag backpacks are designed so you can spread out the load in the most ergonomic way possible, so the load is the least-stressful as possible. In addition, they have straps that allow you to strap it on to the stroller, should you choose.