The underwriter's job is a little bit more complicated than just maximizing the offering for the company. They actually serve many interests including those of their own clients and stakeholders.

A "pop" does not necessarily mean the issue was underpriced. Besides, an increase in share price benefits the founders too

This issue didn't just fail to "pop," it subsequently tanked which suggests that it was overpriced/oversold and while Facebook, Inc. received more cash for the offering, the net value of the enterprise was worth less because of it.

Stock is a form of currency too. For compensation, acquisitions, and more. I think Facebook was out to get all they could, probably still expected a pop (hubris) and the bankers who did the issue let them because they're all shells of their former selves, and they were hedged with the way the deal was structured anyway (greenshoe).

Think of it in terms of taking a cash-out refi on your home with an artificially inflated appraisal. You might maximize the cash you can get, but what's the value of that cash when the actual value of the house is realized and you're at break-even or worse, underwater? Is the bank just "doing their job" structuring deals that way?