"But Gates was in no hurry for Microsoft to take this same rite of passage [going public]. He did not want to open Microsoft's corporate doors to the public. For one thing, the company did not need the instant infusion of cash that a public offering would provide. It was making a great deal of money. Pretax profits were running as high as thirty-four percent of revenues. And remaining private had definite advantages. There were no sockholders to please, no onerous filings with the Securities and Exchange Commission. The only disadvantage was that Microsoft's key employees and managers who had been getting stock options over the years had no tradable security....

"Reguardless of what Gates' wanted, however, his hand was about to be forced. It was only a question of time until the day arrived when Microsoft would have to offer its stock to the public. The 1934 Securities Exchange Act required all companies to register and file public reports as soon as stock had been distributed to 500 or more employees. As far back as 1983, Gates had projected that Microsoft would reach that figure by 1986 or 1987...."

_Hard Drive: Bill Gates and the Making of the Microsoft Empire_ by James Wallace & Jim Erickson, page 320.

For Google, just search for articles around the time they went public.

Microsoft had to go public as soon as they had 500 employees in their stock option plan.