The underpricing pop disappears faster than a fart in the wind. Underpricing in IPOs is direct theft of value from shareholders and employees to investment banks and their clients, and provides zero value for the company.
There's an (also non-exclusive) argument that it's a fair price for the risk that IPO buyers are taking. IPOs do "fail" sometimes, so the pop isn't as great a risk-adjusted return as it looks.
I can think of three non-exclusive possibilities:
* Market power of banks
* Corruption of the stock-compensated managers who benefit from the pop
* Genuine or perceived benefit to the company in further price raising. (Mentioned above)