PNG is portable network graphics. It is targetted for the web, and rather simple (in colors) images. Its compression is very ineffective for realistic graphics as you will shoot with a camera, so the results are nearly uncompressed. As such you can then simply use RAW files, which do add the benefit, that there is no lossy conversion to an RGB colorspace.
The reason JPEG is used is simply that its compression is very good and works very well with more realistic graphics where individual artifacts are invisible to the human eye. In addition JPEG is supported by nearly any device and offers many way to add additional metadata, including color profiles.
No single RGB file format is able to store the image exactly as the camera sensor sees it; there is always some kind of loss in that conversion. As such it makes sense to use a format that has good compression which doesn't really harm the picture in total. If you are looking for a real lossless format, the camera's RAW is the only direction you can go, from which you then can create whatever file you desire.