Because you don’t want tooltips to constantly flicker up when moving your mouse around. This should be a user preference setting, like e.g. double-click speed. (It is on Windows for native tooltips, via the registry.)
Fortunately tooltips are pretty easy to make these days. You don't need Javascript at all to make a nice tooltip. But again, it's a tooltip - it's been done so many times before that it'd be nice to have a native browser version.
Tooltips usually are on links. For example here on HN, hover over "11 minutes ago" in the comment header and it shows the time. Could this be made available to mobile users? Seems tricky. My first thought would be to show it when the user taps and holds. But that also triggers the browsers default context menu for links. So it would make a murky experience.
I think most of the current displays can detect a finger hovering over the display but not necessarily touching it.
It might be a complete disaster though, just wondering how that info could be used.
This is about the best and most intuitive solution you could come up with to this problem. Sadly I believe the Samsung Galaxy S4 was the only phone that supported this without an S Pen so you could simply hover over the screen with your finger. I wish it was a standard feature on all phones.
The first doesn't exclude the second. That's the way how https://xkcd.com/ is doing it. It just shows both.