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.
Steam makes it super easy to get games but installing it is friction?
I am a gamer with an interest in the indie sector but don't have steam because my first experience with that extra platform abstraction - Origin - has put me off. Neither do I want a game as a web page. I want a gaming experience and that for me is still a standalone executable. I even dislike not having a physical copy of digital distributions.
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.