In double entry accounting, the $9B has to go somewhere, or you defeat the primary objective, which is to make it harder to forget transactions by maintaining a number of invariants. The two relevant ones here are [Equity]_t = [Assests]_t - [Liabilities]_t, and [Equity]_t = [Equity]_t-1 + [Profit]_t + [net cash from stock issuance/buybacks]_t - [dividends]_t

The method that GAAP chooses is to move the $9B from cash to goodwill (assuming a cash transaction), which is a transfer from one type of asset to another. The alternative that I think you're suggesting is to move it from cash to expenses for the period in question. There's not necessarily a right answer to this question, accounting conventions are subjective, but I think most people think that the market premium you pay in an acquisition is more like acquiring an asset that will yield future business profits than an expense which, now that you've paid it, will have no future utility to the business. And it's easy enough to look at it the other way if you care to: the goodwill is reported in the balance sheet, and the cash flow statement shows the huge outlay of cash.

I don't know the details, but I'm pretty sure it's tax related. I'll bet companies would love to be able to do that. Because then that company could deduct that capital loss from their income to lower or eliminate their tax burden.