You out if all prople should know that it only needs to br proved beyond reasonable doubt.
Yes, it could be that your luggage was broken when you checked it in already, that‘s true. But because the airline and the passengers are not willing to have to prove and debate and spend money to debate about every single broken luggage the rules have been introduced that if somethig has happened you have to show it directly at the airport. Yes, it is possible that someone broke it on the way from the carusdel to the exit, but I reckon not a lot of people break their luggage right at arrival and then have the odacity to claim compensation from the airline. Back to my original point, the airline is providing a simple and easy way to deal with broken luggage. Could be that this is even regulated by some law.
But as far as I know as soon as you are out of the airport they will not care anymore and I‘m certain they are not obliged to.
And even if you did not see the damage, that certainly isn‘t the airline‘s fault. So why should they care?