Having control over other people’s computing gives you power over them: you can gain from their detriment. It’s not like everyone is uncaring or greedy but even people with good intentions do not have infinite willpower to resist temptation. When the user doesn’t like a change from an update their choice is usually to put up with it. Defending ads in a menu or opt-outs that should be opt-ins in hidden menus is less mental work than learning what an operating system is and that you can use a different one.
By sharing the source code instead you give up that power - if you fail to be good to the users then other devs can work on it without you.