Why am I not allowed to withdraw Retirement Account savings as a lump sum?

You do have the flexibility to withdraw part of your retirement savings after turning 55.

From 55, you can withdraw:

• Up to $5,000 of your Ordinary Account (OA) savings if you are unable to set aside your Full Retirement Sum (FRS);
• Your OA savings after setting aside your FRS; and
• Your Retirement Account (RA) savings down to your Basic Retirement Sum if you own a property with remaining lease that lasts you up to at least age 95. 

If you are born in 1958 and after, you can also withdraw up to 20% of your RA savings from 65.

Your remaining CPF savings are intended to provide you with a steady income stream in retirement. Making a lump sum withdrawal would deplete your savings prematurely and affect your retirement adequacy. Do consider your desired monthly payouts before making a withdrawal.