Obviously nothing prevents people from taking PNG screenshots of JPEGs. You can make a PNG out of an out-of-focus camera image too. But at least I know the format itself isn't adding any additional degradation over whatever the source was.

And in my case I'm usually dealing with a known workflow. I know where the files originally come from, whether .raw or .ai or whatever. It's very useful to know that every .jpg file is meant for final distribution, whereas every .png file is part of an intermediate workflow where I know quality won't be lost. When they all have the same extension, it's easy to get confused about which stage a certain file belongs to, and accidentally mix up assets.