Earlier quoted context omitted.
"your code is a trade secret protected via contracts" isn't enough to replace copyright because the contracts are only enforceable to the signing parties. Let's say you write some software for your employer and they sell the product to several customers with a contract not to distribute it. However, the product appears on some pirate website anyway, and you can't identify who allowed the product to be leaked. Once it…
Yeah, I'm assuming most people here aren't selling proprietary code to customers directly. I assume most professionals here work somewhere that sells SaaS or uses the software they develop internally only.
But there is far, far more in the world that just SaaS or internal only software. Sure, companies are generally shifting towards SaaS (in part because it allows them another way to combat piracy, but also to extract recurring revenue rather than one off purchases), but there's still plenty of software you can go out to the shop and buy. Most of that wouldn't exist without copyright laws, because those developers would have found some other way of making a living instead.