PNG does not compress photos very well. A photo that is 5 MB when saved as a high-quality JPEG may very well be at least 15 MB as a PNG. Also, a lot of cameras (phone or otherwise) save to JPEG by default.
I do wish more people would use PNG where it makes sense, though. The other day I made an edit to an image containing line art that was purely black and white except for the compression artifacts. I applied a threshold so that all the artifacts became either perfectly black or white and saved it as a monochrome PNG, reducing the file size to less than a third, while containing more information and having a cleaner image. I later remembered that I could reduce the file size even more by using indexed colours. In other words, whoever originally saved it in a lossy format actually made it take up more space than needed while also needlessly reducing image quality.
PNG does not compress photos very well. A photo that is 5 MB when saved as a high-quality JPEG may very well be at least 15 MB as a PNG. Also, a lot of cameras (phone or otherwise) save to JPEG by default.
I do wish more people would use PNG where it makes sense, though. The other day I made an edit to an image containing line art that was purely black and white except for the compression artifacts. I applied a threshold so that all the artifacts became either perfectly black or white and saved it as a monochrome PNG, reducing the file size to less than a third, while containing more information and having a cleaner image. I later remembered that I could reduce the file size even more by using indexed colours. In other words, whoever originally saved it in a lossy format actually made it take up more space than needed while also needlessly reducing image quality.