Earlier quoted context omitted.
It's on purpose. The new UI places each post inside a "card". The intent is having you scroll past cards. This makes even text-only posts be similar to image ads posts, and while you scroll you are not aware if the next card is an ad or not. Layouts with heavy information density (as old.reddit.com) are more user friendly, as you they allow you to just scan the list and be done with it. But are bad for ads.
The problem isn’t really the cards. It’s the implementation. If you just reshape shape old.Reddit.com it wouldn’t be a horrible UI/UX. Maybe not great either. Back button is effectively broken with constant reload of front page. 2 links you wanna read? You need to click around carefully especially on mobile if you don’t want the page to reload and show you a different set of stuff.
It's also the cards.