The cause of fuel injectors going bad when not in use is in part because large parts of America contain enough moisture to cause corrosion/rust damage. The internals of injectors are made of various steel alloys that will corrode and or rust when in the presence of moisture.  Gum, varnish and other normal deposits can be removed by cleaning but once an injector dries out and the deposits hardened inside causing a pintle to get stuck to its seat, that injector will become inoperable and in most cases not possible to clean.