Theft and the fact they tend to be in locations (like street corners) where cars may crash into them.
There's a really OG mailbox in my town, it's bent a full 10° towards the house it stands in front of, has classification as a historical relic and all, that has been run into by cars at least 3 times per year since cars were around.
It is still standing.
At one point (around when it got its classification), they put safety poles around it.
Until they got removed, they ended up getting replaced every year because they were all bent to hell from new accidents.
In the US, residential mailboxes are actually suppose to be made to give away when hit by a car. Even the large brick ones in the south are designed to just fall over on a hit.
It's a safety feature for the dumbass drivers hitting mailboxes.
I'd imagine that's a benefit, but the design was probably for structural integrity. Last thing you want is for someone to lean on it, tipping over, and then rolling down a hill or something.