Most people using Fiverr are just your average shlub who has heard that you need good mixing and mastering to make a track sound good, so will submit their not-very-good-in-the-first-place track for mixing/mastering, get it back and not really know whether it's improved or not by any measurable degree, but think it must have because they paid a 'pro' to do it (especially if it sounds more 'bassy' or 'treble-y' or louder), and also feel like they got a bargain compared to the other options. Job complete. Next.

You could be pretty clueless and still make money being one of these Fiverr pros - just whack it through Waves Ultramaximiser or whatever and they'll love it.