people tend to forget that Edo Tensei doesn't just revive someone for free. You need to harvest the corpses and you also need to offer a living sacrifice. While Kabuto said it is a flawless jutsu it really isn't, it need a very long prep time. Not counting that it appears morally revulsive to most, Tobirama itself only used it to enable his fallen comrades to perform kamikaze attacks and still ended up banning even that use (probably because it required the human sacrifice still).
I have an hard time thinking that Tobirama, Kabuto, Orochimaru or Disonasu (!) left some random undead shinobi here and there. I hardly see that happening.