For salaried employees, why does one do a top up to CPF SA? Especially if the cash could be used to possibly generate a higher growth via investments?

When you do top up, you get tax rebate too. Next is SA account get 4 to 5 percent interest, and is interest on top of interest. Risk free.
To get 4 to 5% return for investment, it is consider as mid to high risk already. Why take the risk especially when the market is uncertain at this time?

Quite a few answers provided, so I will jump straight into my perspective.

1) I am aiming for ERS that provides the most cash after I start cpf life payout. Its not that hard to achieve FRS, but there is some difficulty in compounding SA to achieve ERS through the SA balance alone.

2) why ERS? I estimate FRS would provide less than 2k equivalent (in today's dollars) while ERS would provide 2.5 to 3k equivalent when I want to start to draw down on cpf life payout. I am not quite sure that I can live on 2k monthly. I would like to aim for 5 to 7k monthly and so ERS will form the base for half the cash (or less) , and remainder from my investments (be it interests / dividends / selling).

3) risk free 4% is very difficult to beat consistently. I would prefer to diversify and spread my bets between cpf and equity.

4) yes stocks (or S&p specifically) seems to have a good growth rate that compounds exponentially. But there are market cycles, and if you were to depend on it alone, a sudden black Swan like covid could seriously dampen the amount you can draw down for that year. I would rather not risk a life that are good in some years, but absolutely horrible in the down years in retirement.

5) I am thinking of leaving some portion of my investments as a legacy so I would prefer not to touch or count on liquidating it unless I really have to.

As a concluding summary, it's not a one or the other decision path, but employing both would increase the likelihood of a more stable cash flow for me in my retirement years.

That is because they believe in ah gong too much. The SA is a 30 year bond with no guarantee of redemption due to policy risk and a yield that does not reflect this risk accordingly

I have been working for 30 years and began saving/ investing seriously when I hit 30. I have to say that up to 5% risk-free interest that you can earn on your SA is very hard to beat. Plus you can tax relief for the top-up. Only catch is that you can't touch your CPF savings until you hit 55.