People really easily get used to the exact layout of a menu, meaning they don't so much read what the menu says as they remember "the entry midway down the list that starts with G". Flipping it means that they'd have to re-parse all options to find the one they'd need, which nullifies any speed gain which flipping the options would achieve. It also doesn't help that it's a bit unpredictable whether a menu will go up or down before you summon it.
That said: For a spelling mistakes block, this argument doesn't necessarily hold as it's dynamically populated. There may be a case for it to switch position - there also may be a case for it to be disjointed from the main context menu. Google Docs for example has the correction on hover, rather than on right-click.
It's also worth pointing out that just because something would be better in theory, it doesn't mean it'd be accepted by users. For example, ring menus are superior to the classic context menu as every item is exactly the same distance away from the cursor, but outside of games and a handful of apps (like blender), it's practically never used as it doesn't meet the user expectation of what a context menu should look like and ends up causing more confusion.
