There are other ways you can fail to bolt the lid down well enough and the actual secret leaks. You can make all your employees sign NDAs but they still know the secrets. Those employees go and get jobs elsewhere. You can extract some punishment on them for leaking and breaking the NDA (contract law), but once the cat's out of the bag, it's out. If you accidentally leave a laptop on a train, you're done. If a corporate spy targets you successfully, you're done. You can't have copyright on ideas, so if your trade secret is in essence an idea, then people can go and copy the idea.

For a secret that can't easily be reverse-engineered, that means the biggest difference between patents and trade secrets is how long you expect to have a monopoly for. For patents, if you are granted one, then you get a monopoly for exactly 20 years, which for almost any idea is enough time to make a lot of money if it's a good idea or at least get enough of a head start. But once 20 years are up, that idea has been in the open for 20 years already. For trade secrets, the number of years depends on you. Some companies can make it for more than 20 years. Employee retention helps. Spending lots of money on information security helps. Not letting Chinese companies through the door who pretend to want to buy yours, do some due diligence and then back out once they figure out a few of your trade secrets, also helps.