This isn't really true.

In California specifically, Labor Code Section 2870 limits an employer's claim over inventions or products created a) outside of work hours, b) without using company resources like your work laptop, and c) not related to/competitive with the company's business.

Even outside of California, it's extremely rare for companies to try to claim ownership of side projects that check all of the boxes above. Legal action is expensive and bad PR unless the employee is clearly infringing or building something competitive.