Because Reddit's native search is completely broken.
If I'm looking for something specific I've seen before I use redditsearch.io if I'm looking for a comment by a specific user I use redditcommentsearch.com if I'm looking for de novo results I use Google site search on Reddit.
People use Google to search Reddit, not Reddit.
I have found Google to be the absolute best way to search for tweets on Twitter. Twitter search is attrocious.
I do search for things on YouTube directly, but that's still Google Search.