Right, you would look at similar things that your potential carpenter has built. You would also try to do that with programmers. But a good deal of the things programmers built are team built, or proprietary, it's difficult to know what they've created in the past. Having them work on a simple task beforehand is a way to normalize and make sure every programmer you're looking at is on a level field.
Also, unlike a programmer, a carpenter can't simply copy+paste work that someone else did and claim it as his own. It's a lot harder to fake tangible work in real life than it is to fake intangible work on the internet.
They can point at a house and say "I built that" even if someone else did, and how are you going to know the difference? The same way as you verify that someone in software did the work they said: put in the due diligence. That part can't really be avoided.
