They're supposed to be exactly as stable as required by law for their intended use, using the minimal amount of labor and materials so satisfy the requirements. Making them more stable using more money is easy, but getting the calculations just right requires years of training.
I see what you mean now. My point is that even if there's a minor screwup, the architect probably won't be prosecuted for anything because the building won't fail catastrophically thanks to modern standards which have substantial margins of safety built into them. It really does require criminal levels of negligence or extreme circumstances that would almost certainly save the architect from prosecution to have a building collapse on you.
On the other hand, a minor screwup in software is far more likely to cause catastrophic failure because we just don't know how to workably build large, robust systems out of code.
Nit: meeting the requirements of law isn't barely collapsing; that requirement has so many safety factors built-in because the building code has to approximate so much. The approach isn't that dissimilar from how that "anybody" you mention would build their bridge that doesn't fall down: by guessing safely.
