Definitely something that happens from many causes.
In this town, the Town's road specification, directly conflicts with the Post Office specifications for the installation of a mail box on a post, its been a long time since I've read and compared the 2 but the town clearly states there shall be no obstructions, with dimensions from center line of road, or within the right of way from center, and of course the post office states a distance from the pavement or roadway, certain height etc, so all the mail boxes are in violation on town roads, no one enforces it. Probably not wise to set a W or H shape steel beam or column, you may be liable, as it can't shear off. I have seen steel like that used in a county north of here on a road, several people did it, probably still there.
Prior, and long time neighbor lady, I believe had trouble with alcohol throughout here life, more so after he husband passed away, she was great without the alcohol, I used to clear the driveway in winter and generally tried to be helpful whenever possible. She was still driving and though sometimes she may not have known where she was or what time it was, she still drove, one morning 6 am sharp, I got home from out of town the night before, planned to sleep in, I hear this crack/bang! She backed right into it and it was offset way far enough from her driveway across the lane, southern yellow pine is brittle, so the treated post just sheared off, and both our mailboxes on the cross piece on top, were literally upside down. A relative stopped in, to help right the situation and I said to him, Marge's drinking ain't doing you, I or the mail box any good LOL ! I dug up the concrete base eventually, and the hole was more than I figured, kept running back to the hardware store for bags of concrete and the darned clerk thought I was stealing, using the same yard ticket ! I informed him otherwise, lousy $2 bags of concrete, really ? LOL !