You're right. They don't.
But I really don't think Apple is trying to directly target PC/Console gamers with this platform (at least not right now). The much easier target market for them is the casual / free-to-play gamer. They already have millions of people playing games on iPhones and iPads around the world. If they can get these people to convert into a recurring revenue stream via subscription that's a huge win for Apple.
Also... Apple rarely stops at generation one of a product. Arcade will be refined, updated, and iterated on once they start to figure out what works and what doesn't.
The casual free to play gamer is not going to subscribe to a (probably) $10/month subscription. They're used to free/sub $2 games. $10/month will seem crazy to them. And it is for a pool of 100 indie games. That $10 can get them a sub like netflix, apple music, etc. It goes a long way to getting HBO, etc. Casual gamers aren't going to pay anything for a game subscription.
They have millions of people playing free games. You're right if they can convert them into paying $$$/month for a tiny pool of indie games it's a huge win. OTOH, if they work out a way to covert lead into gold cheaply and efficiently, it's a huge win, and at least that's a theoretically possible goal.