My riding pants (and jackets) are much heavier and stiffer than my skiing clothing. Skiers and snowboarders seldom find themselves traveling through rain at, say, 70 mph, and they don't need to protect against abrasion from road surfaces during crashes.
Quality ski gear is also designed for ease of movement, and for operation (zippers, adjustments, pockets, vents) with cold, numb fingers. Riding pants are constructed for static function in a seated position--nothing more. For example, some riding pants have a relatively tight cuff--keeps it in place while you sit there all day on your bike, prevents it from flapping and from admitting wind-driven rain and spray, but not so easy to overlap over bulky ski or snowboard boots.
Does this make any difference to you? Maybe not enough to justify the price of purchasing separate clothing for each. I find them substantially different in use, and I use my riding gear for riding, my active sports gear for active sports--hiking, climbing, skiing, etc. YMMV.