I suggest app inventor to add Admob module, and then share profit with users. It will be a win-win situation. Or just add the admob as a paid module.
Many users have asked the same question, and many users have answered with same answer, "App Inventor is a learning tool and doesn't need things like AdMob as App Inventor was created solely for learning purposes." But if you still need AdMob you can take a look at Kodular (https://creator.kodular.io).