Why are Agencies so wide?

Following yesterday's announcement that a Carlyle fund is in default over its repo arrangements on Agency RMBS, Bloomberg comments on the agency/treasury spread:

Yields on agency mortgage-backed securities rose to their highest relative to U.S. Treasuries in 22 years as banks stepped up margin calls and concerns grew that the Federal Reserve may be unable to curb the credit slump.

The difference in yields, or spread, on the Bloomberg index for Fannie Mae's current-coupon, 30-year fixed-rate mortgage bonds and 10-year government notes widened about 7 basis points, to 223 basis points, the highest since 1986 and 89 basis points higher than Jan. 15.

Why? There are a number of reasonable explanations. Firstly and most obviously there are more buyers than sellers of treasuries and more sellers than buyers of agency MBS. Why does no one step in to arbitrage the spread? Because there is little risk capital in this market that is not deployed, and/or anyone with money is waiting for things to get worse.

Next, even if we do accept that an arbitrage relationship holds between treasuries and agencies, (1) the liquidity premium on agencies is high; (2) the credit spread on agencies is increasing [since as the agencies lose money and become more highly leveraged, the likelihood of government support must decline somewhat]; and (3) regulatory risk in the mortgage market makes it unclear what the cashflows of the underlying assets will be anyway. (Remember even if the agency guarantee holds good, a reduction of principal on the asset pool causes prepayment, thus regulatory risk effects the optionality of the pass through.)