Lots of obvious reasons: It wouldn't fit, it would make the tailgate heavier, and you couldn't have the cover partially closed or closed with the tailgate down.
I don't see a vent in this pic of the rear drive unit, the white tubing could lead to a remote vent or be a low pressure air line, or something unrelated.
Something like a kayak would not be a problem since it's an aerodynamic, (hydrodynamic) shape anyway but a piece of plywood could be an issue if it went past the front windshield. However a standard 8ft plywood would not stick out that far and should still be in the laminar flow of the vehicle.
You could add a "T" fitting in the high pressure line and put a quick connect air hose fitting on it for use in an emergency. I'd just keep a good portable air compressor in the truck instead.
I used to run Interco Super Swampers on my truck, insanely loud on the road, sounded like an airplane propeller, but saved me from a long hike back from the wilderness a few times. Wore out too fast on pavement for a daily driver though.
Higher voltage and temperature speeds degradation of the electrolyte and physical changes to the electrode.
This paper gets deep into the weeds but the EE video above explains it well.
Electrolytes for high-voltage lithium batteries