I'd say for a number of reasons they're different. Plumbing, while probably useful at some point in your life, is just one activity. Programming is NOT just one thing. It's closer to woodshop (which I hope is still in schools...). Another difference I think is that you can create something bigger with it, something that scales. Last but not least, I think it's inherently fun and fulfilling to create something out of nothing, but I'll admit that it's not for everyone. But neither is woodshop or gym class. I'd also say that teaching some programming along side math could have some benefits to learning and motivation.

Programming is a much more general skill than plumbing. Don't just think Java enterprise apps for a Fortune 500... programming is fundamentally about learning how to use computers to their fullest. The ability to explore and create with computers is a powerful skill that complements most of the potential careers that a child may pursue in the future. Pretty much any science or engineering field will benefit from programming experience. Increasingly other fields are starting to move in directions that will make those skills more valuable.

Computers are just at the very beginning of revolutionizing the way that human society interacts. Why would you not want your children to have an edge?

Programming is a fundamental concept like arithmetic or persuasive argument. It is about turning abstract human thought into machine-executable algorithms. Children who learn to program will grow up smarter and more productive than those who don't, even if they never code for their job.

In addition, the number of jobs that requiring programing skills is exploding and will continue to grow. The basic idea of automating information tasks is useful for many, many tasks.