No mention of PNGs? I don’t usually go to jpegs first for screenshots of text. Did png have worse compression? Burn more cpu? I’m sure there are good reasons, but it seems like they’ve glossed over the obvious choice here.
edit: Thanks for the answers! The consensus is that PNG en/de -coding is too expensive compared to jpeg.
PNG is VERY slow compared to other formats. Not suitable for this sort of thing.
PNGs are lossless so you can’t really dial up the compression. You can save space by reducing to 8-bit color (or grayscale!) but it’s basically the equivalent of raw pixels plus zlib.
PNG can be lossy. It can be done by first discarding some image detail, to make adjacent almost-matching pixel values actually match, to be more amenable to PNG's compression method. pngquant.org has a tool that does it.
There are usage cases where you might want lossy PNG over other formats; one is for still captures of 2d animated cartoon content, where H.264 tended to blur the sharp edges and flat color areas and this approach can compensate for that.
PNGs of screenshots would probably compress well, and the quality to size ratio would definitely be better than JPG, but the size would likely still be larger than a heavily compressed JPG. And PNG encoding/decoding is relatively slow compared to JPG.