Why don’t aircraft undergo contaminated-runway landing-distance and rejected-takeoff certification testing? All aircraft have to undergo testing to determine how much runway they need in order to safely land and come to a stop, or to safely reject a takeoff.

They do not, however, have to undergo this testing on anything other than a perfectly clean, dry runway.  Instead, to get landing/RTO distances for contaminated runways, they multiply the dry-runway distance by some arbitrarily-determined factor (precisely which factor depends on what the runway is contaminated with - water, slush, ice, wet snow, dry snow, sand, Teflon, Jet A-1, ping-pong balls, whatevs), and then everyone hopes that this actually gives them enough of a margin for landing on contaminated runways.

Why not simply run landing-distance and rejected-takeoff certification tests on contaminated runways, as well as on dry runways, and directly measure how far it takes to stop on a wet or icy or whatever runway?
Baseline performance data was developed some years ago by an FAA test program called the Joint Winter Runway Friction Program to develop mathematical models of the effects of the different types of surface contamination.  Manufacturers apply the models to develop their own aircraft's performance correction factors for various contaminated runway conditions.

Otherwise, as Ron comments, the manufacturers would have a massive test burden to develop customized data for a huge range of contaminants, and would have to go find the test conditions somewhere for each, adding eleventy jillion dollars to the typical development program.  The mathematical models, along with the safety factors built into the calculations, do the job just as well.

This Advisory Circular talks about it.