Most web pages "guilty" of this are owned and operated by corporations and they're doing it for a reason: visitors apparently trust websites more if they look professionally styled, so they prefer those over simpler looking competitors at first sight. Market pressure forces websites to polish their UI so it becomes attractive, usability is only an optimization (if you don't have a significant number of users due to the website's initial impression, it's not as important to optimize for maximum profit).

Some early movers got away with crappy visual aesthetics for a long time (like my former company), but today an 'RMS-style' (or old craigslist style) website would not stand a chance upon entering a competitive market.

This works a bit like we judge other humans, perhaps: based on appearance, before they even open their mouths (and maybe that's the reason why we have better haircuts now but whether we are better characters than a few generations ago, is debatable).