If not put into Kiwisaver, I suspect many of the funds would have ended up going towards a deposit for investor or owner occupied housing. The tax settings for financial investments vis a vis both investor and owner occupied housing being what they are.
As it is, there is not much incentive to contribute annually to Kiwisaver more than the minimum necessary to get the annual tax credit - especially for the self-employed. Even then, the current National Government $512 annual tax credit reduces as an incentive when the balance grows to over about $30,000.