Both of those things are absolutely possible, the problem is that it separates the dev and testing steps. Its difficult to explain why exactly this is a problem

Both of those things are absolutely possible, the problem is that it separates the dev and testing steps. Its difficult to explain why exactly this is a problem, so I'll give an example. Imagine your developing a game on your high end PC, you run it and it runs at like 10 fps. Clearly something is wrong, thats a bug that needs to be fixed before you do anything else. Now imagine it runs fine on your high end PC, but in testing, it runs at 10 fps. The issues are the same, and should be treated the same, but in reality they rarely are. The testing issue would certainly be addressed, but not quite with the same urgency.