When you block the wheels in loose snow or gravel, it piles up in front of the tire and provides a fair amount of friction. This is usually the fastest way to stop, and one of the reasons that gravel pits along corners in motor racing are so effective.

That said, the point of ABS is in the rare event that you have to brake full power, the system automatically help you do it at a near optimal (slightly skidding) without additional input, and you remain full steering ability.

If you don't have ABS you'd need to train that emergency stop ability on a daily basis to even come close.