No reason at all, except the finance business is not very innovative in areas like raising capital for companies. It would be easy to have a structure like an ETF where brokers could get more issuance from a company if they had excess demand for shares.

Underwriting does get you what it says, a guarantee that they will take the whole allocation, which was the original reason for underpricing. There have been a few cases where underwriters lost a lot of money (eg the BP floatation in the UK), but usually the underwriting fee is pure profit, especially if you can get the launch undervalued, and there are usually options that make it pretty hard to lose money (as with Facebook).