Frequently because they're forced to, by the threat of litigation and/or a smackdown from consumer-protection organisations. There's too often a window where Apple won't acknowledge there's an issue but your warranty's expired so eh...maybe they'll make an exception for you? Or maybe not.

I'm looking for something a bit more systematic. On an iMac Pro that starts at a shade under £5000, it wouldn't hurt them to make the default warranty 3 years and offer an extension to 5 (I mean, who's throwing away these machines after 3 years, never mind one?).