Because people aren't using the correct Internet to play video games.
Also after mw2 they started using different online architecture. Mw2 was the best game and there was no lag. After that is when it got bad. Bo1 was the worst. Bo2 is the best since mw2 but its not even close to being as good as mw2. And mw3 is in the middle, between bo2 and bo1.