Liability doesn't make sense for a lot of software. It's not like electricity where it's always potentially dangerous, civil where mistakes can cause death and dismemberment en masse and so on.
Basically software liability is just rolled up into the relevant market. Writing facebook games? It doesn't really matter to society if you get it a bit wrong. Writing pacemaker software? Then you're governed by medical device regulations.
Software's application to problems are so varied, so wide, that it's insensible to apply a single liability scheme to it.