[notahacker]

Successful sites that are poorly designed are outliers - early movers in big movers. It's not quite true to say that they succeed in spite of their design (a clean-looking, more usable Plentyoffish would get fewer ad clicks) but if you're competing with them then a great UX is going to put you at less of a disadvantage when it comes to attracting and retaining visitors, because the concept of dating sites being free isn't a new one.

Likewise, if you're Amazon you can afford to send users contemplating buying away from your website with sponsored links because they will most likely come back to your big name site to complete the purchase. If you're not Amazon you probably don't want any distractions for the users not geared towards upsell.

To some extent their designs are stuck in a local maximum because their relative ugliness is part of their brand: if Craigslist tidied up their design they'd probably see a big rise in bounces from people confused by being sent to listings that don't look like Craigslist.