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?