Apple went all-in with USB-C on their computers (starting 5 years ago with the macbook pro). And that was a pretty unpopular decision at the time.
Now that the mobile phone industry has standardized on USB-C, they want to stick with lightning? Are they doing it just to be contrarian?
If they continue to avoid the widely-used standard, they reduce the likelihood a consumer will buy their overpriced first-party peripherals. This hardly even counts as nefarious: it's openly their brand to dismiss the benefits to the user of interoperability.
That, or being lightning-only is simply more profitable. Public companies (like it or not) have a fiduciary duty to maximize shareholder wealth.