western hauler type bed

Bucky, sometimes words have different meanings. Look at the standard features on this trailer and you will see "dovetail" mentioned. Which is the drop at the rear of the trailer to make loading cars easier.

big tex dovetail car hauler

Here is another manufacturer that lists "dovetail" as a standard feature:

gooseneck flatbed dovetail

Using a word differently than someone, doesn't make the use wrong, it is just a different use for the same thing.

We all can certainly call something anything we want, but sometimes we would be wrong. See clicky.

Dovetail clicky

Farmers often go out on very rough roads and fields with gooseneck trailers, so they need the extra clearance.

5th wheels rarely see such rough conditions so your standard pickup bed is typically fine.

to roll hay off the back easier, for wind to direct under the trailer

Bed sloped down at the very back of the truck bed, downhill on a slope seems like a good reason, but if it was necessary I would think a regular truck bed would hit really easily.

I call it a dovetail because when the back of a flatbed trailer does that I call it a dovetail

Do you mean why does the bed slope down? That’s for maximum clearance to a trailer when making a turn that is a sharp downhill.

Or do you mean why are the corners clipped at 45 deg? That is for clearance to stock trailer landing gear which is often a single heavy leg in the middle.

Or do you mean “dove tail” which is widened out like a Y- like the joint cabinet makers use? Don’t buy one made like that :) Lol

Most likely it to mount the hitch lower so the trailer will be level when hooked up.