That’s because the houseguests actually entered the house last Friday – every year the houseguests go into the house long before the first show. That gives them time to get settled in and get used to the mics and cameras, layout of the house, etc. before we see them on the live feeds. During that time producers are talking to the houseguests a lot over the speakers in the house, so they don’t really want us seeing that either.
Conspiracy theorists say that they use this time to memorize their scripts and try out their characters. 😉