Dboy
It is because a bunch of conditions in the OS need to be met for automatic updates of apps to occur, e.g. device is in idle/the app to be updated is not in use. People won't be happy if their app gets abruptly forced closed due to auto updates.
The notification is to alert that there is an update, so users that prioritize getting the update over their current browsing sessions can do it manually. If it is ignored, automatic update will eventually kick in later when conditions are met.