Craigslist became popular when that was the clean look for a website, and then never bothered updating. Network effects in action.

Temu offers people the ability to save money. If your product is "X, but cheaper" you can have a worse UI than X.

Amazon is a hideous looking website too and always has been. Ebay is similar too in that aspect and plenty of others.

They were established when hideous websites were common changing would cause uproar and possibly alienate a large group of users. Look at Googles web page just a text box on a blank page, no way you are launching with that UI in 2026.