Many, either they forget where they leave their back up, they can't retrieve the coins from their back up (which could be mistake when they wrote the back up), lost their hardware wallet, lost their phone etc.
I don't have such experience, but I have read someone else's experience that he lost access to his wallet because the cellphone he was using was damaged by flood and he didn't have any backup to his wallet seed phrase. It was just carelessness to install a wallet on a cellphone and not back up the wallet seed phrase, even though in this day and age we can't just secure all our data on one device, especially when it's a cellphone that we often carry everywhere which is very vulnerable to being stolen, lost, or dropped and causing us to lose all data on the device. It's better to back up important data, especially the seed phrase which is important access to the wallet that we have.
Let's imagine if he backup his seed phrase on paper inside a bottle or steel plate, there's a chance the flood can sweep away the bottle and steel plate,
even cars can be sweep away by flood. You have to think about a backup that can prevent against that, either place it at higher place or hide under something that you always carry without making people know if you carry secret information.