The DOJ argues that consumers are being shortchanged on privacy and security as a result of a monopoly, even though these are two of Apple's primary marketing points. One specific criticism is that the company has intentionally stifled cross-platform messaging.
Apple sells itself to consumers on security and privacy, and one of the building blocks of that ecosystem is its App Store policies. Should Epic's antitrust lawsuit succeed, the "walled garden" may be breached.
Facebook and Apple have been going back and forth over changes made to iOS 14 for the benefit of user privacy. Facebook appears to be shifting to an offensive posture as it mulls an antitrust lawsuit.
There have been rumblings about antitrust lawsuits targeting Silicon Valley's biggest companies. The first has finally emerged. If the suit is ultimately successful it could break up Facebook.