They are! There are US antitrust cases going on right now against Google (x2), Amazon, and Meta/Facebook. Some links that may be of interest:

• FTC Sues Amazon for Illegally Maintaining Monopoly Power
• U.S. and Plaintiff States v. Google LLC (2020) – trial began on September 12, 2023 (this case regards search, I think)
• U.S. and Plaintiff States v. Google LLC (2023) – complaint PDF (this is a second case against Google)
• FTC v. Facebook