[bmm6o]

This is a community that values a high signal to noise ratio and generally eschews small talk, a la nohello.org. Congratulating someone for learning something does not advance the conversation.

It also has a low tolerance of what it perceives as reddit- style in-group signaling via repetition of a common meme (xkcd, in this case). Again noise vs signal but also suspicion of karma farming.

[JumpCrisscross]

Bingo. Same reason “congrats” comments are downvoted. A comment page full of empty congratulations is thoroughly uninteresting.