Earlier quoted context omitted.
Apple blocking iPhone browsers without Webkit (excluding Chrome and Firefox). Android not exactly making it easy to change the default browser for average users either tbh Relevant XKCD https://xkcd.com/1118/
My theory on this is it has something to do with battery life. I can't think of another reason they'd limit browser engines.
Apple was in deep troubles when IE was all the rage, you needed IE to access your bank's site, your company's intranet, so many site that were strictly IE only. Microsoft gifting them an implementation of IE was a mild grace, but still wasn't enough. Not having a browser compatible with 99% of the web was an issue pushing people away from Macs.
They learned their lesson, and used their advantage in the mobile market to make sure no other browser ever puts them in a similar position.
Google has a slightly different issue, in that for instance a strongly privacy focus/ad blocking browser taking the android world by storm is nothing good for them.