Its a problem compounded by many things.
1. Airlines charging for hold luggage now just encourages people to bring it onboard in the hopes it won't get offloaded to avoid the fee. Airlines also have confusing onboard luggage allowances, a handbag plus a suit bag plus a laptop plus a cabin bag not totalling more than 7kgs altogether. I mean who is really going to think about things that much? It should be one bag, period. Whether that's a laptop, handbag or whatever doesn't matter. One bag.
2. Too few staff to actually scan for oversized luggage at gate lounges. Gate staff are usually only there long enough to disembark one flight and send another out. Otherwise they are onto the next gate.
3. Tight schedules mean there just isn't an allowance of time for cabin crew or ground staff to be offloading bags, issuing baggage tickets and adding bags to the hold. Pushing back an aircraft on time is always a major concern.
4. There is no requirement to test bags in the test unit so passengers just ignore them. Passengers often get argumentative when they are informed their bags are oversized. They will give the staff member concerned endless grief about how they always bring this bag and that bag onboard and this is the first time anyone has objected to it.
5. Inconsistencies across airlines and the network. One person may do their job with the bags but the next will think it's too much hassle and just let it go through leaving it up to someone else to break the bad news.
6. Most people don't consider if anyone else can fit their bags into the overhead lockers. As long as their stuff is in there that's as far as the thought goes.
Most onboard luggage is something that can be checked into the hold. It's overnight luggage not laptops or purses or other breakables. Why people want to save $10 on checking bags when they will probably spend more than that on coffee and a snack on any given day I have no idea.
Personally I think an easy way to solve the problem would be to have smaller x-ray machines at passenger screening points or a size gauge that the bag must pass through to get into the machine (have larger ones for business deliveries into the airport), if the bag won't fit in the machine then there's really no arguing about it.
