From a user's perspective it wouldn't be a phone worth buying if those SDKs weren't provided. Why do we have to accept Apple's framing that SDKs are a gift bestowed upon developers rather than an essential part of the software ecosystem that users are buying into?

Third party apps can't exist without the SDK, but the iPhone wouldn't be viable without third party apps. Who makes money in this arrangement has nothing to do with what's fair and everything to do with who has more power.