probably  because  spv buys AAA asset by the money collected from investors.  A company with all  asset rated AAA , so it s just AAA

一般的公司，公司发行的债券会被评级，公司本身也会被评级。 SPV同理,CMO类似发行债券， SPV类似公司（由于SPV的特殊性，其实也可以理解成是作为债券MPS的评级）
SPV不但是卖方，有时也是CDS的发行方（风险承担方），所以要被评级

SPV 是要出去 忽悠人的， 所以 肯定是 AAA.

There is the same question (Q43) in 2009 FRM Full Exam Practice Exam1 from GARP. The solution is as follows.

In a CDO transaction, the Special Purpose Vehicle are special entities of financial institutions and are usually AAA rated. The SPV and institution are legally distinct and credit quality deterioration of the financial institution does not affect the SPV. In this case SPV counterparty risk is low, which is desired by the investor.

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强大！