iOS has no shortage of games, so that doesn't seem like a problem. And maintaining their own graphics API lets them fully integrate it into the rest of their hardware and software stack without the baggage that comes with a cross-platform standard.
iOS is a pretty big market, I would argue that they have lots of games despite not supporting open standards, not because. Gaming compatibility on laptop/desktop macOS is pretty piss poor. (Although Blizzard has always been really good at supporting mac).