Like you I'm a gamer, and like you this doesn't appeal.

However I can see exactly why people are doing this. There have been a big explosion of indie games recently and that can be put down to iOS and Steam. Both make it super easy to get games.

Anything that reduces friction is going to be very appealing to game makers. And installing Steam is friction. If playing your game simply involves visiting a link - thats pretty easy. As HTML5 matures, as more people use modern browsers, and as Flash becomes less reliably installed on browsers, expect to see a lot of casual games moving to HTML5.