I thought about one for my old truck. But I can't really see any real benefit Vs. the tents I already own.
I can set those tents up or down, and I am free to leave and come back with the truck without having to strike down the tent & all the gear inside.
To set up the tent in the bed, I have to unload everything I brought out including coolers, camp chairs, back packs, firewood, ect...
Tent on the ground means I can leave the items in the truck until I decide I want them off.
It gets you off the ground by a few feet, but if you get enough rain to really flood a proper placed camp site, you should be in your truck anyway moving out of the flood zone.
Are you worried about water, or reptiles or what as the reason you don't want to sleep on a tent on the ground?
As mentioned, in the winter with the air gap under the bed, you will be pretty cold too.
It's a free country and you can spend your money as you please, but I'm not seeing the benefit to it...
Except as humor - former member Napoleon Dynamite (famed for the "Throw the rifle down the driveway for the battleworn look") wanted to go on a trip with his girlfriend. He was going to put up one of those tents inthe back of his pick up and imagined his girlfriend riding back there in the 70mph winds... napping or resting...

I'm pretty sure the high speed highway "winds" would have either broken the poles around and on her in the back,, or pulled the tent out of the truck at speed making a ... Big Mess...
BIGGER_HAMMER