Why do people always say that it is not profitable to invest in Singapore stocks? is it true?

It is profitable. Just not as profitable as US or HK stocks.

Singapore companies are all old school type, unable to expand overseas successfully and command big market share. Look at ST engineering, they have overseas expansion, yet they are still a tiny tiny fish, unable to compete against Lockheed martin that produces the one of the best military tech.

Big startups don't list in SG too. So opportunities for good companies in SG are getting lesser. Those that remain are "dividend" stocks, or dying companies like SPH.

As what others mentioned here, low liquidity, low volatility and also the nature of the stocks in Singapore. Financials and REITs aren't really dynamic industries that come up with life-changing innovations that can swing share prices.

The circuit breaker system and share halts are also rather turn-off. I'm pretty sure if Tesla were to be listed in Singapore, everyday sure get queried by SGX hahaha

Because they prefer the volatility, rather than painting a fence.

Singapore stock exhange is considered backward when compared to the other major exchanges. The low volume and lack of market makers discourage investors too.