They do it because it makes more money. They've measured the amount of time spent (and ads served) to people with and without the feature, and they found that,

They do it because it makes more money. They've measured the amount of time spent (and ads served) to people with and without the feature, and they found that, averaged across the user base, the feature increases revenue. So everyone gets it. At least, that's what every ad-supported platform thinks they're doing. There can be other factors, especially in large process-oriented companies like Google. Like, an ambitious PM wants to be able to point to a feature they added in their annual performance review, so they do some sloppy measurements to show that their dumb feature increases revenue even if the change is just noise. You can see why such a PM might not want there to be a way for people to disable the feature, since that metric could clearly show how unpopular it is.