Lets just zoom into a single use case. The ability of the user to buy a 3rd Party watch that integrates with their phone:
* Apple doesn't allow 3rd Party watches to send text messages. The Apple Watch is allowed to do so.
* Apple doesn't allow 3rd Party to take actions on notifications. The Apple Watch is allowed to do so.
* If you want to use the internet on your watch, you must: 1) install a 3rd party app, 2) keep that app open. Closing the app closes the connection to the internet. The Apple Watch does not have this restriction.
* 3rd Party watches cannot detect if you are using your phone. This means that they will notify users of notifications even if the user is looking at the notification. The Apple Watch does not have this restriction.
* Apple does not have ‘interprocess communication’(IPC) like Android.
* Apple restricts making 3rd Party App Stores. This makes it difficult to make a community of people making watch faces.
All points come from Pebble's blog [1]. This is just a single type of integration that Apple intentionally makes difficult, there are many others (e.g. 3rd Party Photos App, ...)
[1] https://ericmigi.com/blog/apple-restricts-pebble-from-being-...