Because it's a software middle an that injects itself into search results in a frustrating way. I'd love it and would use it all the time except I constantly run into it in a way where instead of just going to the link, it tries to force me to sign up or log in, and blocks the actual link with it's nonsense.
The product itself seems great but they're customer acquisition process is way to aggressive and they have to be manipulating Google to get the SEO over the actual original source of the match.
This is how I feel, too, I use it on occasion and it's actually nice as a digital pinboard, but I despise every other thing about pinterest. Now I use kagi and just have it blocked pretty much.