"...Surplus masks are generally outdated/expired..."

Plus it is hard to get the proper sized mask when you buy on-line, which is probably how most folks obtain such a mask. Bad fit equals worthless mask, no matter how fresh the filters. My agency used to pass out those compact USGI M-28 "Grasshopper" masks for riot duty. No fitting, just passed them, in their cute little green pouches, out like candy. If you ever actually put the thing on, chances are that it wouldn't fit (plus the filters were probably way out of date)...