Why does the ATIS say "read back all instructions"?
Due to the ongoing runway incursion epidemic, US ATC must hear every aircraft read back certain instructions, including callsign. If you don’t read them back correctly, they’re required to repeat the instruction until you do. No matter how many times it takes.
When this rule was first adopted (or at least enforced), a shocking number of pilots couldn’t give proper readbacks. This caused a lot of frequency congestion and other issues, such as making an aircraft on final go around or holding departures just because someone taxiing toward that runway wouldn’t properly confirm they’d hold short of it.
Things have gotten somewhat better as pilots have learned, but it is still common enough of an issue in practice that they won’t take the reminder off the ATIS. And since some other countries either don’t have or don’t enforce this rule, it may never go away.