Why is the lock in period for whole life insurance plans so long?

Simply because you're not supposed to.

They're protection products meant to pay a sum assured upon death, disability, or illness.

They're not savings policies.

Also the cash vaue on a whole life plan is meant to adjust your coverage for inflation. That's about it as well.

But whole life, die whole life. If you're ever considering terminating your policy early, don't buy it.​​​

The reason for the long lock-in period of whole life policy is because insurance companies pay their agent a high up front commission. This doesn't mean the insurance policy has a high commission or insurance agent earning a lot of money, it's just the mechanics of accelerating the commission up front so that new agent is able to make a living. This is not a problem since whole life policy is meant to be a long term instrument and you should not be thinking of surrending it ideally. If liquidy is important for you, then insurance policy is a wrong product, as they should be considered mainly for protection, not investment return, even when there is cash value. You should be looking at pure investments like stocks, ETF or unit trust, for liquidity.