I was thinking about that as well. ;-)
I thought of a female hex the same size as the nut, welded to the angle and a retainer over the top to stop the nut falling out ... or just tapping a M10 thread in a little bit of 10mm plate and tacking / bolting [1] that on there. ;-)
However, it's quite handy to be able to rotate the nut as then you can set the position of the eye and then nip it up. Or, if I want to make the eye movable it might be easier to let the assembly rotate without the nut undoing.
In hindsight I also quite like the simplicity / flexibility of using a std nut / bolt assembly. Easier for field repairs or mods etc.
Cheers, T i m
[1] Using two countersunk machine screws from the top of the angle before fitting it to the trailer.