Tragedy of the commons. Say Company A contributes. Company B does not, and therefore reduces its costs and extracts value from the commons (in this case open-source software) at lower cost and greater profit. That encourages Company A to do the same.
Even if Company A's officers and board agree to contribute for any of various reasons - sustainability, PR, etc - voluntary contributions are the easiest cuts to make when there's a downturn in profits and the stock market must be appeased with prayer and sacrifice.