I'm pretty sure they do get softer?
Take metal bar, bend one end, fairly easy.
Take the same metal bar and compress it (without letting it buckle) pretty much impossible.
Push it ar any angle between the 0 and 90deg mentioned above and the modulous is proportional to the angle.
Coil it into a spring, and the flatter the coils (i.e. the more its saged through plastic deformation/creep) over time the softer it gets. It's also shorter. So the spring is both a bit softer and a bit shorter, so the result it it bottoms out easier.
If you don't believe me, come and have a look at my car, after 45 years on the same springs the drivers side is about 1" lower than the passenger side!
Towbar mounted bike racks put a large moment on the towbar, possibly more than a trailer/caravan which has its own wheels.
Towbars have a maximum nose weight, usualy 50-75kg, whether thats from bikes or from a trailer makes no difference, apply a given acceleration (say 2G ~20m/s2 vertiacely) and the towbar feels the same form 75kg of trailer or bike rack, about 100-150kg.