Why do you suggest we don't use CPF to pay for housing mortgage?

CPF OA has 2.5% in returns, much higher if you invest them. Cash sitting in banks usually have interest up to 2% and using them for mortgage will incur less opportunity cost.

The exemption is when you are able to outperform your OA investment (e.g S&P500 index) when you use cash. Outperforming the index is hard and many professional investors have failed to do so.