Because it worked so well last time

The alchemists of Wall Street are at it again. 

  The banks that created risky amalgams of mortgages and loans during the boom — the kind that went so wrong during the bust — are busily reviving the same types of investments that many thought were gone for good. Once more, arcane-sounding financial products like collateralized debt obligations are being minted on Wall Street. …
    “All of this seems like a fairly quick round trip,” said Manus Clancy, a managing director at Trepp, a research firm that focuses on commercial real estate. “You are seeing a fair number of sins being forgiven.” 

New York Times