Because companies can't find where to (profitably) invest their money. All other thing being equal if a company which never gave dividends, anounces a dividend, its stocks will plunge slightly; as it's a signal it is plateauing. Of course, companies may make foolish acquisitions like Facebook and Yahoo but since these are not profitable investments, they will be in financial trouble later.
When BRK/a or BRK/b (which don't pay dividends) are priced "worthless" in the market someone let me know about it so I can buy.