Not saying this is always the case, but I've noticed many crews will automatically just say "we didn't get any - catering error" without looking into it much further.
I can think of an example recently on a transcon PLH flight that should have the snack basket. They didn't offer any snacks, so about 45 mins before landing, I asked if they had any snacks. SD was quick to say "no, catering didn't give us anything."
I'd happened to notice a full bag with the snack basket items was in the front of the overhead bins, as it is on some narrowbody flights and pointed it out to the SD. She looked a bit embarassed and said "oh, I hadn't noticed them", despite the fact that I'd seen her open/close that overhead bin multiple times during the flight. Not saying this happens all the time, but I've seen a few cases where there was a closed bag of snack basket items that was visible either in an overhead bin or galley, and they weren't offered to passengers.
In a lot of cases, it may be catering errors or cost cutting, but I've also seen cases there the crew just doesn't offer the items or "doesn't see them". This probably doesn't help the cost cutting, as if a crew doesn't offer the snack items and they get returned, AC thinks they are being too generous and overcatering, and cuts the quantities.