Why don't DSLR cameras default to png instead of jpeg?
Its force of habit and good enough. People were using JPEG's in their digital cameras before, so they stuck with it, and its plenty fine for viewing (and it produces pretty small file sizes as well). You're free to turn to some other compression method your camera supports.

Note that the RAW files often contain a lot more information than what a PNG would have anyway (like a bunch of stuff about the sensor), which can be exploited in programs like photoshop for editing, so from that perspective, you may as well have the RAW rather than the PNG (especially since the RAW is rather space efficient to begin with). You can also put in things specific to your camera there if you want. 

Note that many cameras have the option to shoot RAW+JPEG so you simultaneously get both, which is good for viewing (use the JPEG) and editing (use the RAW).