The same reason Gmail can't work with end-to-end encryption--they want to advertise at you based on message content.
I highly doubt there is any government intervention in FB's business strategy, but there seems to be plenty of cooperation after the business decisions are made. (The same is largely true with Microsoft, Google, and yes, even Apple.) It's not really a conspiracy, or a matter of paranoia--it's been very widely reported for a while now, and people just generally don't seem to give a shit (myself increasingly included).
EDIT: I do care. But I think (a) people need to take privacy into their own hands, since companies will never be incentivized to do it and (b) we've entered a new cultural era, where the levels of privacy enjoyed in the past are no longer socially normal.