It's not so much that they think they are funny, more the absolute build up. It's presumptuous.

Saying things like "you might need to sit down" sets up the line for disappointment. There's no way it can be as funny as it promises.

The pun is sound, but the hyperbolic framing let's it down.

And if it was so funny your legs go weak, then it doesn't need the build up in the first place.

I think this type of social media framing belongs in the same category as videos that have text all over them saying "wait until the end!!!". The video loops around and you think, "what did I miss, was that what we were all waiting for?"