Apple didn't think games would fly on the iPhone. They were wrong.
They are not a gamer company, but they have built a really solid platform, with very low entry requirements for development. Yes, there is an annual subscription fee for the developer program, and this is a requirement for submitting applications, but this cost is relatively small. Everything else you need is free (not counting 3rd-party toolsets and libraries).
Whilst PC gamers can tweak their hardware, what about PS3 and XBox360 gamers? How much tweaking can they seriously do?
And how many games are available for iOS compared to the Wii? I have and iPad and a Wii. I enjoy both, they are different experiences, but I only have about 20 games for the Wii. I have played many more times this on the iPad.