I don’t even think it’s the stiffness of track and crit bikes, it’s the relationship of the rear tires contact patch to the saddle. The further forward the rear tire is placed, the impacts from road bumps send the saddle more directly up, into the rider. The longer the chain stays, the more bumps will rock the rider forward instead of directly upwards.

I have no credentials or proof, but I do have opinions and I’m on the internet.