It's one of the reasons why longtime Mac users are disinclined towards non-native apps. Most don't bother to reproduce these behaviors, and so when you as a user go to reach for these features that you have muscle memory for and they're not there, it's like hitting a brick wall in the middle of your workflow and makes the app in question feel basic and unrefined.
Oh and of course those shortcuts are activateable by other applications to simplify app integration.
Alt+underlined letter with no menus opens opens whichever menu maps to that letter
After which any [unique, as they mostly are] underlined letter activates the press for that menu item.
There is no equivalent to the speed of this in OSX, even with third party software.
Very very happy to be proven wrong on this