We do, but you don't generally see seasonal shortages because they project production rates and deliver enough product to keep the warehouse shelves filled.
Vendors in America obviously don't overstock Chinese products at the end of the year because inventory on hand is taxed in that year. They let it dwindle at that time and re-stock in January. My college accounting teacher told us he hated being a young accountant in December because he always spent the last two weeks of December as the less tenured guy doing inventory in clients' warehouses.
Just in time inventory control means you're out of stock if regular shipments stop arriving!