I mean that’s literally the joke. Haha you thought you were going to see one thing but you saw another.

It’s just another version of 2girls1cup or tubgirl or goaste … Ok maybe us millennials are fucked because all those examples are heinous AF compared to a little rick roll - but I mean, same concept.

Just subversion of expectation, many a joke is built on a version of that - I’d say it’s ultimately what a lot of humor derives from.

Maybe it’s not funny, it might not even be a joke and more of just a meme but if you are searching for an explanation there is one for you.