Which is straight up what jmbwell was complaining about, as it breaks their workflow such as tapping on the top to scroll to the beginning and everything else they mentioned.
Some applications have neither a beginning nor an end, so scrolling to the beginning makes no sense. At best, you could make "scroll to the beginning" get back to the start of your navigation, but then your scrollbar wouldn't let you go further.
(also if you have both horizontal and vertical scrolling, you're still left with no horizontal options)