They actually don't, sadly. Most companies limit the risk using non-competes.

It's very hard to secure code. If someone wants to steal it, they can usually figure it out.

It's much easier to limit their legal ability to profit from stolen code. Most companies don't have IP that's very valuable on the black market. If you stole Zoom's compression algo, for example, it would be hard to profit without openly starting a new company and violating your non-compete.