That's because they are made from cheap plastic.

I'd really love to know why on earth they don't use better materials on all of their high-end phones.

I mean, sometimes they get it right and use a nice rubberized material. And then they blow it again, like on the Galaxy S where the plastic back-over is a harsh insult to the otherwise excellent hardware.

Is that rubberized plastic really so much more expensive?

You don't need to reach to the expensive glass or metal to make a phone feel less like a kids toy!