It's how people deal with video (adaptive bitrate streaming). It also used to be how people dealt with images in the dark days of content rewriting middleboxes. So it's not that nobody's thought about the idea: it's that it's way harder to do once it's a question of really changing behavior rather than just being more aggressive with a quality/size trade-off knob.

Also, mobile friendly versions of sites are at least as much about the user interface and rendering speed as about reducing bandwidth usage.