They don't do it out of goodness of their hearts, which is why it's more solid than relying on goodwill - Microsoft simply has an offering that depends on that for certain high profile clients.
Frankly they should still be getting sued for the way Edge and Cortana are bundled.
People should be free to use other browsers and accept higher power usage, or "privacy risk" without Apple forcing their own browser engine, which limits functionality of all web browsers on iOS.
What Microsoft did by bundling IE with windows was far less egregious. Could you imagine if Microsoft forced all browsers on Windows to use IE as a webview? Holly shit people would be rightly pissed.
Apple does this specifically to prevent competition from web applications so that developers have to use Apple's app store, where Apple can charge the developer/customer money. It's spelled out in the DOJ lawsuit against Apple.
https://www.justice.gov/archives/opa/media/1344546/dl?inline