Who says they do not? All manufacturers of cameras specifically state their sensor size dimensions in millimeters.
If so, where?
Yes, the DSLRs, whether full-frame, APS-C or some other size, nearly always state it in their specs, but I've looked pretty hard to find this for point-and-shoots, and have come up empty. I think I'm not the only one who can't find it, as I seem to recall a column by the NYTimes' David Pogue on this very subject.
Say manufacturers, isn't it time for full disclosure?