If you add tread and use softer rubber snow/ice/slush, you might have too much friction on other surfaces, ie generating excessive heat, then if you land your snow tires somewhere with hot temperatures, possibly more blowouts...just thinking out loud. Cars rarely travel to different climates. Cars also don't have reversers. The added complexity in managing spares etc...it's probably a mix of logistical and cost issues that outweigh possible (small) benefits.