Why isn't PNG preferred over JPG for being lossless art quality yet?
I don't know if this is the real reason, but here's one possibility: because it's unnecessary.

It's easy to say "JPEG = lossy = bad", but JPEG doesn't just destroy image quality willy-nilly. What the format gives you is a slider with "small, low-quality" at one end and "big, high-quality" at the other. You can easily reduce the byte size of a PNG image by encoding it as JPEG, and if you use the high-quality end of the slider, you needn't lose any visual appeal. Although normal JPEG is lossy in the information-theoretic sense, a high-quality JPEG appears identical by the naked eye.

Many image hosting sites will replace images by lower-quality JPEG files not because of the storage cost but because of the bandwidth cost. If you're serving millions of views of these files every day, the cost of sending those extra bytes is pretty big, even though the cost of storing them is tiny. And those bytes don't just cost money: they also cost load time for your users on mobile networks. These sites will have targets for how long their pages take to load on slow connections, and reducing the quality slider ever so slightly makes a big total difference.

In summary, JPEG encoding can be lossless or lossy, and it can be as high- or low-quality as you want. Using PNG everywhere instead of JPEG would be costly for image sites and would make you wait longer for your art files to download. Choosing a high-quality JPEG file can give you something that looks just the same as the PNG without being as "heavy".