If STI ETF is a horrible investment, why do people still do DCA on it?

I would not say that STI is a horrible investment. There is just better choices out there that could outweigh the Sti returns.

Why I would not want to invest STI,
1. Sti is too concentrated on the financial markets

2. I can better diversify with a global etf. It can be found in robo advisors. The barrier to entry to foreign markets is higher than Singapore markets. For more diversified choices, I would choose global than Asia for dbs digi portfolio.

3. I have less knowledge on global markets than on Asia markets. So it is easier for me to go into Asia markets if I want to. So for balanced overall portfolio, i choose global

Why people still buy Sti?
They want more Singapore exposure. Or simply put, they are confined to Singapore. (sorry for being rude but this is what I observed)

People still do DCA on STI ETF because they believe time in the market is better than timing the market even though the growth of STI ETF is kind of flat. Personally I would recommend investing in a diversified portfolio with different assets classes with low correlations.

Our banks have been marketing STI ETF very agressively over the last few years, creating awareness on such products, and claiming that it is easy to set up, and the 'track record' of STI ETF. I guess the bottom line for them is the fees that they will earn.

If you are looking for 6% annualised return, I would suggest that you look into REITS directly, instead of through roboadvisor. In the long run, the fees that roboadvisor take from you might eat into your annualised return.

I also think STI ETF performance is terrible, that's why it consist less than 5% in my portfolio. I treat it as dividend portfolio for long term, together with my REITS. I may eventually replace STI with REITS.