I can imagine the pilots not hearing or not paying attention to the FA announcements at the beginning of the flight, but usually a pilot gets onto the intercom early in the flight and mentions the destination at least once IMO.

I've been on flights where one of the early announcements naming the destination (IIEC by a FA) was strong but then was immediately corrected.

I'm also a bit puzzled how the pilots could have boarded without noticing the destination that was posted at the gate as they were verifying that they had arrived at the correct gate, but of course this wouldn't apply if they had just stayed on board the aircraft from a previous flight. When the GA lets pilots on board, wouldn't there be some verification of flight number and destination as well as the identity of crew members?