I asked the same question about my axles, until I remembered the tongue weight.
My TT is designed for a max of 7000 lbs on the axles and 700 lbs on the tongue for a total of 7700 gross.
In the real world, the CAT scales showed the heaviest I have ever loaded it was 5900 lbs on the axles and 700 lbs on the tongue.
If I were a boondocker or off-roader I might opt for a higher rated axle, but then I do not think a standard built TT is really designed to hold up to those rigorous conditions in the first place.