As a folding bike must accomplish more than a regular bike, there need to be compromises somewhere, such as fewer speeds, smaller wheels, getting subjectively less for the price, etc. For most people 5 or 6 speeds is plenty enough under most circumstances. The majority of people answering that yes they have done it, or it is of course possible, have experience, skills, bike parts, tools and auxiliary parts, such as bolts and nuts, and they enjoy engaging in the effort as a project. Unless one wanted this to turn into a project where one learns bike mechanic skills, for an average person without such skills it makes no sense, both economically and in terms of a project that gets completed in time. In fact, this can easily turn into a project that never ends.