The trouble is the people in the middle. They used system UI where possible, but then rolled their own where Apple didn't have a good solution. If you recompile those apps against iOS 7 SDK, your app looks like junk, and it's a lot of work to fix it. If it's not a super-profitable app, are you really going to go back and get new art done, or are you just going to keep it the same, by using old XCode or something.

The other trouble is that a lot of devs don't use AutoLayout, because it's historically been very hard to get working right. And transitioning your app to it hasn't had many benefits in the past.