If reits are so good, why dont people all just buy reits? since it seems to have guaranteed returns

There is never a return guarantee, only are the dividends distrubuted when things go well.

Money can additionally be lost with a REIT price drop.

I feel the best way to invest is not into single REITs but S-REIT ETFs, that one of Lion-Phillip is better focused onto Singapore property REITs than the other available S-REIT ETFs.

And not everybody buys REITs because there are other important investent vehicles, like passive indexing ETFs.

If you really believe that all reits are good investment (guaranteed returns), no one is stopping you from putting your money in your own mouth lol.