Comment by ralferoo

"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's there, your contracts are worthless as nobody who downloads it from the pirate site will be bound by your contract. You need copyright laws to prevent subsequent distribution.

Without copyright laws, you'd then end up in the situation where people who paid for the software then are bound by the contract and would have to keep paying, and those who just downloaded it from a pirate site would be completely fine legally and could use the software with impunity. That would create a disincentive for anybody agreeing to buy the software and entering into a contract in the first place, essentially killing the industry.

You might not see a problem with that if you believe in Open Source, but most licences require copyright laws to exist to protect the software freedoms so that people don't just take the software and close source it.