Can anyone explain why we need to return the money we use from CPF OA for housing when you sold the property you use it for?

When you use your OA monies to purchase a home, you're borrowing that money from yourself.

If the money was left untouched, it would have earned interest.

But since it's not there and was used to finance your home purchase, you'll need to refund the amount used plus the interest that it was supposed to earn when you sell your house.

This "accrued interest" is still your money as it goes back to your OA account. This means if you sold your house to buy another house, you can use the OA money that was just refunded to purchase your new home.