Both major app stores prohibit explicit sexual content in apps. Not restrict β prohibit. There is no age gate, no rating, and no paid tier that makes it permissible, and enforcement is not limited to what ships in the binary: an app that is a front end for an adult web service can be removed for what the service does.
So a platform that wants to be on a phone has to be a product where explicit content is either absent or firmly walled off from the app experience. That is a real constraint on what the product can be, and it is why the apps that exist skew toward the companionship end of the category rather than the adult end.
It also means an app can vanish. Removals happen, they happen without notice, and a user whose access was through the app loses access entirely while a web user does not notice.