I usually attribute this to the idea of reuse: by definition, every software problem we are solving is "new", because if it wasn't new we'd just use existing so

I usually attribute this to the idea of reuse: by definition, every software problem we are solving is "new", because if it wasn't new we'd just use existing solutions and there'd be no team. (Now there are people that insist on resolving problems, but they put themselves into the "new" category by choice). New problems mean unknown ways to fail === hard to estimate