Because it's invasive.
The user consents to sending you information when they hit Send(explicit). They can remove any incorrect or unrelated information from the text box before they do so.
Especially if they paste in to the text box and inadvertently paste the wrong thing(be that sensitive info or similar).
The experience is all in the context, and since this is only general behavior for customer service chats (where you're expected to send everything you type and the other end is simply trying to help resolve an issue), it's not really a realistic issue in terms of privacy. I agree that if facebook messenger started doing this and showed the other person what you are typing, this would be unexpected and potentially unwanted behavior, but the likelihood of unwanted behavior in a customer support context is extremely rare.
How would most people know to complain about something they don't know is happening?
How many of those 10k people even know this is happening? I suspect, given the other comments here and the fact that this article exists, that it's a very small number. Try informing all 10k people first, then count how many 'may not have an issue' with it.