We used to see this a lot back in Oklahoma where I grew up. Bumper-mount hitches don't have a lot of weight capacity, and they're often overloaded. On top of that, the bumper-mounted hitch is usually higher than an under-bumper receiver, so the trailer tongue would be far from level when hitched up.
When you consider that pickups didn't have any requirement to even HAVE a rear bumper back then, they didn't have to meet the same Department of Transportation strength requirements as a passenger car bumper. I haven't checked recently; do pickups require DoT 5mph bumpers now?
Engineering: Finding complex solutions to simple problems you didn't even know you had.