Earlier quoted context omitted.
Well, one technical reason is that WhatsApp groups — unlike Telegram groups — use encryption. So Facebook doesn't know what goes on inside them.
That's just a theater. If anybody can join a group, there must be a way to distribute the keys to those who joined. Anybody can get a key then, including Whatsapp/Facebook.
But a Facebook employee would have to join the group and they wouldn't have access to older messages. It's not like with Telegram, where all the content, including old messages, is stored in plain text on the server.