I can think of some great reasons.
Back up cams start at $8 on eBay or Amazon vs a GoPro that is hundreds.
Back up cams can be permanently mounted and wired into an in can screen.
Nothing you are trying to see under, or behind you rig is worth seeing or recording in HD.
Backup cams come with IR night vision for seeing in the dark(seeing stuff like animals in the woods)
You could mount 4 roof cams, a front and rear cam, mirror side cams and even the belly cam for less than one GoPro.
I've used my GoPro and cell phone in a pinch as a temporary backup cam, but both most be charged and the GoPro only lasts a few hours on a charge. So keeping an eye on a trailer on a long trip is much easier with the true backup camera.
Theft! Nobody is really trying to steal permanently mounted tiny backup cams. A GoPro would vanish in a heartbeat.
I would suggest to just use any of these action cams. like a go pro or any other brand. These include everything you need! They are optically stabilized! record internally. And videoquality is way better compaired to Rear view cams.
There are action cams with 4K. The ones above have qvga. This is roughly 10 the resolution! I don't see a reason to use a cam with such a bad quality.
PS: I personally will go for a different approach:
https://youtu.be/tB-S1fV3nZM
The newer ones can follow up to 55mph.