Yeah no one bothers to help in securicar or start escorting it. All it needs is someone to think "Hmm, I'm free and I'm not wanted. Lets chase the securicar and jack his car when he's at the last pickup collecting the money, easy 30k-50k" and he gets his 30k-50k. Most of the time cops don't bother saving the securicar, even if they do respond they'll try to blow it up rather than save the securicar. They just don't have an incentive to save the securicar at all. If the van survives or doesn't it doesn't matter to them.