Some companies like Manley, Retro, A-Designs, and a few others use NOS when it's feasible. For instance, they'll use 5670s, 6AL5s, and others that are not in high-demand but go with modern-stock for the 12AX7s, 6V6s, etc. Mic designers are going w/ less expensive and more available NOS options like EF800, 5840, 6AK5. As has been mentioned, it's just not possible to source large quantities of the popular types. Once you screen out the truly quiet and healthy pieces, your cost goes up many fold (most of the popular types have been picked-though already). Then, you need to have several thousand that are from the same production period, otherwise not all of your units will have the same sound. It would require a full-time employee who is an expert in the biz just to source the tubes and, even then, there's no way of stocking enough pieces for any company that is producing in the thousands. So, it's not really an option for them.