@Shanny8 Because it's usually a lighter format in terms of memory use and a massive international site like webtoon based solely off of image files wants to keep server costs down.
Also, generally artists don't want the HD file up online as there's been a gaggle of bots that swipe content off the web to make physical prints and goods off of them (see the "this site prints stolen images" tshirts). Anything that's up in HD not watermarked is at risk of getting monetised by shady companies, and the easiest way of stopping that is to put up an image that's average internet quality, with medium level jpeg compression, 72ppi and around 1000-1300 pixels on it's longest side if we're talking about a classic page format. It's an extra layer of protection for your art.