Certainly not defending LLMs here, don't mistake with that.

Humans do it too. I have given up on my country's non-local information sources, because I could recognize original sources that are being deliberately omitted. There's a satiric webpage that is basically a reddit scrape. Most of users don't notice and those who do, don't seem to care.

Yes, the most likely reason the model omitted the signature is that humans reposted more copies of this image omitting the signature than ones that preserve it.