FYI, it's against the law for private-hire drivers (like those who use Uber and Grab) to pick up kerbside or taxi stand passengers without prior bookings. But apparently some still do that. According to The Straits Times, since 2013, the Land Transport Authority has investigated 35 alleged cases of private-hire vehicles picking up customers illegally.
Passengers aren't the ones complaining, but the National Taxi Association has raised this issue with the authorities, who are taking enforcement actions. Several taxi drivers also claim to have seen private cars waiting at places like nightclubs, shopping malls and roads leading to airport terminals, in the hopes of getting passengers.