I wonder why cameras don't do PNG yet ? Isn't that loss-less compression ?
PNG is indeed "lossless" as long as you stay within PNG's limitation (32 bits/pixel max if I'm not mistaken). PNG was not meant to be used in the professional printing world (no CMYK for example, you only get RGBA with PNG as far as I know).
The main issue is that PNG works best when there a lot of identical pixels (and ideally long stretches of identical pixels). When it's not the case, PNG is actually quite awful.
For example if you take a picture made of only black & white pixels and containing huge area all black or all white: PNG will compress it fine, because it shall realize there are only two colors used etc. You'll get a really tiny file.
Now create a picture of the same widht/height but where every single pixel is using a unique color: PNG's compression will be totally awful. The size of the resulting file will be bigger than JPGs (even with JPG compression set to high-quality).
In photography you rarely have two pixels of the same colors and hence typically PNG can't compress these pictures efficiently, which is why PNG is not a good choice in this case.