Unfortunately, the thieves don't have to be all that sophisticated to make the money unrecoverable. What they typically do is open a phony bank account in the name of the victim. They receive the refund into the fraudulent account, and they immediately withdraw it. By the time anyone notices that the refund was fraudulent, the thief is long gone, and there is no money in the account to reclaim.
It's not just the IRS who gets ripped off this way, by the way. Thieves use a similar technique to cash stolen credit cards. The thief will open a phony merchant account and a phony bank account in the victim's name. They will run the stolen cards on the merchant account and deposit the proceeds in the phony account. They are able to withdraw the funds before the fraudulent charges are noticed and reversed, and the processing company that the merchant account was opened with ends up eating the loss (or passing it along to their legitimate customers in the form of higher rates).
Preventing this kind of fraud has costs. There are monetary costs associated with putting antifraud measures in place, and there are costs to the customers in the form of having to wait longer for their financial transactions to go through. Basically, everyone involved (the banks, the IRS, etc.) has to balance the losses against with the costs of preventing them. When fraud is rare, it's cheaper for them to eat the loss than to prevent it.
If fraud starts to become more common you will see the institutions involved put into place additional checks to prevent improper transfers. Tax return fraud has become common enough that the IRS has instituted prevention measures such as requiring information from previous year tax returns in order to receive your refund. If that doesn't curb the problem, then they will probably add more measures, and perhaps they will slow down the payment process. However, they probably won't ever get the fraud losses down to zero because that would mean both angering taxpayers by delaying refunds and spending more money on fraud prevention than they save in avoided losses.