Those two stores are not the exclusive avenue for customers to play games. Developers may choose to bypass those stores altogether and release their games on disc if they choose.
Good point.
Aren't all PS4 discs AES signed?
I believe that (barring a hacked system) there is no side loading of games that aren't Sony-blessed.
Sony blesses them, but the ways in which a consumer can obtain those blessed discs is much more varied than the App Store, and Sony doesn’t oversee the sale of discs. You can’t rent or borrow apps, can’t resell them, can’t trade them...