Tech giants quietly buy up dozens of companies a year. The Biden administration is finally noticing.

A soaring number of mergers and acquisitions, many of them never publicly announced, is overwhelming antitrust regulators, a major problem for the Biden administration's hopes of intensifying scrutiny of corporate power centers like Silicon Valley.

Already this year, companies across all industries have sought to buy or merge with others worth at least $92 million almost 3,000 times - roughly 40 percent more than before the pandemic in 2019 - according to federal data. Regulators at the Federal Trade Commission, charged with upholding competition laws alongside the Justice Department, are warning they are unable to adequately review this magnitude of activity...