Why?

Because programming isn't carpentry. The difference between an average programmer and a great one is that the average one will build what you ask according to blueprints provided. The great one will come up with new ideas for how to make the product better, drive improvements throughout the team and so on. Building sites are hierarchical - by the time the carpenter turns up the plans are finalised and can't be changed so easily, but software is always in motion.

Also, when I look back at the hires we had that didn't work out, "didn't appear to care about the job" is a common theme. Carpentry is a physical job out in the open where everyone can see if you're working or not. Programming isn't. If you're in front of your desk it can be hard to tell if you're working, or goofing off, and the much greater flexibility given to developers vs carpenters feeds into that (e.g. if you're goofing off temporarily we'll cut you some slack because you're a valuable employee).

So in an environment where supervision is less rigid, the gap between most effective and least is high, and it's hard to tell when people are working, having an interest in the work (it doesn't have to be "passion") means you're much more likely to work out.