Because one nothing in it for inno, it's all about making money.
Two, you don't go backwards you go forwards.
Three, if did that you would lose a lot of players and nothing to make new players join as building city from scratch and attempting to take cities of folk is hard enough without it being made harder with no tasks or farming villages.
The one great idea Inno had was Hep blessing and they did it once saw there was no beneficial benefit in it for them and got rid, great benefit in it for the players would have brought more in but nah players would have spent less building cities up meaning inno would lose out on players spending more.