The reasons are quite obvious. First, you'll have trouble telling a high quality JPEG compressed photograph from one stored in PNG. Second, photographs don't really need layers. Third, a 6 MP PNG image would be gigantic. They would take forever to send over the internet over the average internet connection, and you'd only fit what, 20 images on a 1 GB memory card? Something like that.
Actually, a bit more. I just did a test, and i ended up with 13 MB for a 6 MP image.
Don't get me wrong though, PNG is good, but it's not practical to use in all situations. I use PNG for my own graphical works, but i convert them to JPEG when I upload them to the internet.





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