The information in a DICOM file is a huge huge superset of the information you'd typically find in a PNG, because the amount of metadata you want to tie to a medical image is enormous (e.g. what day, what scanner, which scan, which part of which scan, which radiology tech, which doctor, which patient, with which contrast agent, in what orientation). Even if you did contrive a way to pack that metadata into a PNG file (which is possible), you'd still need a way to unambiguously interpret all that metadata, and know that everything else in the healthcare world is going to interpret it the same way. That's one of the things that the DICOM standard covers.