The problem is that IF they did this, there is nothing to prevent you from loaning or selling the original disc, nor to restrict the next person from doing the same thing. People would quickly exploit this and either rent the game instead of buying or just trade in the disc after it was installed.
If you want the convenience of not having to put the discs in the drive when you want to play, but digital...but then you can't trade/sell/loan the game to anyone else.
Personally, I have bought ALL of my software this gen digitally. I tire of losing discs, scratching them, or loaning them out to people who never return them. It's worth it to me to just buy it and never have to worry about it again. Then again, I was never much of a fan of buying a game for $60 and trading it in to gamestop for $10...so, there's that...







