Its crazy, but sometimes people have non-Apple devices. There's a decent chance none of the baggage handlers had an Apple device on them when they were handling the bag, or they were only around the bag so briefly it was between the airtag's chirps.

There's a good bit of separation between the cabin and the cargo hold. I imagine it could be pretty difficult to get a read from a low power tag deep in a bag in a pile of other luggage with several other big metal plates between.