You can't really make robbers less aggressive ("You don't want to hand over your stuff!? OK, get lost then!" would make for very poor robbers (in two senses of the word)).
You can do other things, such as reducing their frequency, reducing the size of the groups (2-4?), increasing the chances of detecting them so you can avoid them (a bit unrealistic as robbers you can easily avoid won't rob many, but could be argued for in the name of "game balance", or could be a game setup parameter).
In terms of it being a "survival tax", there aren't exactly huge amounts of sinks for your wealth... You can get your gear back by recruiting a posse and kill the robbers, but that carries a cost that's probably higher than the losses to the robbery itself (but masterworks equipment is hard to come by, and getting rid of a some of those Njerp weapons you've collected probably doesn't come as a problem).
How realistic is it that a group of robbers looking for travelers are able to set up an ambush for spotted travelers without being detected? Fairly realistic, I'd say (fortunately, I don't have any real life experience of it). Real world robber victims often get robbed through ambush. The robbers hide and one of them steps out into the path of the victim while the others surround the victim.
The unrealistic part here is UrW's zoomed out map travel and that NPCs don't know how to hide, so the ambushing is abstracted. Also, real world robbers tend not to try to rob heavily armed groups of travelers outnumbering them, while UrW robbers fall into that counter trap every time (once you've found them).
Different games have different target audiences. UrW is not aimed at the power fantasy fans, but more towards the simulation/rogue like audience. Life isn't fair, and nor are those kind of games. That said, the games shouldn't be overly unfair, at least not without a good reason.
In terms of unexpected encounters, I'd take robbers over bears, wolves, and Njerps any day. I've had characters killed by wolves and Njerps, but never by robbers (badly beaten, yes, but I had a choice in fighting them). With bears I've been lucky (my current character survived with serious injuries as the bear attacked (and killed) the newly purchased dog, allowing my character to crawl away (literally)).