Because it is interesting? Because it is one of those things which fits in the frame of mind of many a hacker-type? For the same reason that some of us wear hiking boots in the server hall, just in case a mountain might spring up there?
> a nuclear war is probably not survivable
Of course it is, just look at how Japan came out of one. Would it be a pleasant experience? No, of course not, but the same was true for Hamburg, Tokyo and Dresden during the fire bombings, London during the Blitz, Stalingrad during the beleaguerment, the northern part of the Netherlands during the "hunger" winter of '44-'45. We like to read about war, watch movies about war, sing songs about war, create and look at art about war but few people actually want to experience war.
> at least 5 years in an underground bunker
As long as only "normal" nuclear weapons are used a few weeks would probably be enough, long enough for the fallout to be washed away by rain. The next step would be to leave the area and head for "the hills" - or any other area which was not directly attacked nor in the fallout plume of an attack.