Anon, when you send any diplomatic mission there is a small probability of catastrophic failure. Even if the mission is completely unopposed or isn't even headed to a place where defense is possible, some missions fail. I don't know the percentage; some say 1 in 100, but in my limited experience I think it's a bit higher than that.
If you are sending out 20k to 50k thieves at a time, that small chance of catastrophic mission failure translates into a giant loss of units. It takes a LONG time to make that many thieves, even with substantial sovereignty.
For this reason most "accomplished" thieves tend to split their attacks into smaller groups and therefore must attack less well-defended targets.
In addition, most large alliances and experienced players have methods of tracking attacking diplomats back to their home base. I can only speak for my alliance, but we are successful in identifying the attacker at least 50% of the time for single attacks and virtually all the time if attacks are repeated. If you do this to a large player or alliance, the consequences can be dire. Most players who have alliances don't want to put their alliance in jeopardy by doing this; most players who are not in alliances have a sense of self-preservation that would also prevent them from doing so.
I'm not saying that thieving from large players can't be feasible, just that there are complexities that you may not have considered. We are seeing more large-scale thieving than we had previously, but I think that the factors that I have outlined will limit the scope of such activity.