Because the costs involved are tiny compared to lost productivity based on developer salaries and, more obviously, because these same companies spend millions on coffee stations, roof decks, alcohol-focused parties, etc.

People who think cost drives open plan office choices seem to think corporate management are too stupid to ask, what is 10% of our total salary base for these workers. It makes no sense. People have been measuring productivity and morale in knowledge work for a hundred years. Of course they can directly compare cost savings on floor space with metrics correlated with productivity lost to worse workspace, and tie it to the bottom line.

The observable spending habits of companies suggest cost utterly cannot be a driving factor here.