What if they don't?

a couple of reasons.
One is NHTSA/FMVSS regs says the sum of trailers GAWR's shall not be less than GVWR.
The other reason is one I'm personally familiar with. I've towed with class A cdl combined for 11 years moving my own wheeled construction equipment and material in several different states.
The first thing DOT does is move you to the impound lane and red tag the trailer. You have some options.
One is find a licensed vehicle alterer or modifier to come on site and add a GVWR certification tag to match GAWR's. Very expensive.
Another is then If the load is over GAWR's then you will be compelled to unload or rearange the load on the trailer to comply with GAWR's. If a second vehicle is dispatched to pick up your overload that is also expensive.
Anyone should check with their own states DPS/DOT or a weights certifed state trooper for their interpetation of how a truck and trailer are legally loaded.