Mobile browsers don't support it. I've never been clear on the reasons, except maybe that there's a tension with the carriers over airwave bandwidth. Honestly: there's a Javascript interface in mobile Safari to access the location sensors and, IIRC the accelerometer, but the camera is offlimits.
On the other hand, the HTML interface is a mime-multipart form and a file upload - it's only convention that says "upload images only" - and the mobile platforms are very particular about access to files on the device. iOS for sure won't allow one app to touch the files of another.
How come? Mobile browsers don't support it (for security reasons?) or it's slow/eats up precious bandwidth? Or am I misreading you completely.