Ok, many developers, especially at the beginning of their careers, do like to introduce new things into the projects when they believe those things could advance their careers or even simply sound cool to talk to the peers about. But this is not the reason for the unreadable web.

I think the reason is simpler, more like fashion: most people who are in a position to play with fonts and colors have absolutely no idea what they are doing and make decisions by copying somebody else who they perceive as cool, and there is always pressure to make it look cool, so it spreads.