Because the page source is more annoying, thanks to thinks like Vue and React, which generate the DOM on the browser, rather than building an actual web page on the server.

Take Twitter for example. The elements don't exist until the page loads, and none of the classes have any useful meaning, which makes user styles impossible, and user scripts need to rely on a DOM observer or a timer to find attributes or (even more often) element text to locate some elements to hide or modify.