1. We have a cultural context that tells us how hard it is to produce something like a song, or a painting. There's a reverence for it. And so people tend to back off of their estimations for the amount of time required to produce it.
2. Software isn't physical. You can hold a big book in your hand say CRAP! that must have taken a long time to do. But you're just looking at the result of a program, if it's done well, it's a tiny streamlined user interface. The result of one button click could have taken thousands of programming hours and thousands of lines of code but you would never know it. Software obfuscates complexity from the user in a way that someone looking at a huge huge painting or a big book doesn't run in to as much.
So if we're talking about the efficiency of natural heuristics in estimating things, I think software is still slightly more difficult to estimate than art. But I agree with your overall point.