If this was true in general all social networks would be interoperable.

You may value a product like that more but imo it’s not the way it works in practice.

It is true in general but for users, not for corporations that profit from walled gardens.

Correlation != causation. Furthermore, the business models of a phone manufacturer are much different from a social media platform. Most phone manufacturers still make the bulk of their money directly from the end-user paying them cash (either for the device itself, or associated services), where as social media platforms primarily make their money from renting access to their captive audience to advertisers.

What you’re describing is true, but the iPhone runs general purpose software. They don’t have to give away their own features to their competitors and people who aren’t their customers, and it’s more valuable to them as something they took the time to develop as a point of differentiation to give their customers a better experience than they would otherwise have.

An iPhone can already send files or stream media to all products provided you negotiate the protocol by which you do it and the software is available or you write it yourself. YouTube has Google Cast built into it, which you don’t have to use with an Apple TV because AirPlay will work, but if you say wanted to put video from your phone up on a Samsung smart TV, it’s an option. If you want to transfer files from an iPhone, you are not lacking in options, but if it’s to another iPhone or a Mac, AirDrop is just one of those options.