Because the existing games are usually on their last legs long before then and then further demanding people buy an expansion for a game they don't play at full price wouldn't see many people coming back especially at a premium. Would also start splitting the playerbase and leave departed players alienated and possibly not coming back. MW3 is an isolated case because Activision did not plan ahead adequately and I wouldn't expect them to repeat this again. If they wanted to do down this avenue, it should've been done years ago and done properly, however the series was too busy pissing around with boost jumping, wall running and going back to World War 2. I don't believe they've made a game that offers a confident foundation for them to build upon either but MW2019 would've been a good starting point and then they go and follow it up with a highly inferior game.
It's definitely possible for this to happen - Siege, Fortnite, Apex and CS maintain strong playerbases and even grow still to this day but COD always struggles with this respect and until they produce a game that has lasting potential and a cemented design, then they will forever be releasing annually rinse and repeat. If only they released COD Online worldwide and supported that...