Not every app works the same way at the airport, and you usually only find out once you've picked up your bags.
Grab is the only app allowed to pick up inside the airport grounds. A dedicated zone is signposted as you leave the international terminal, with a stand you can't miss. You book, walk two minutes, and get in.
Bolt and inDrive work too, often at lower fares, but their drivers can't come to the terminal. You have to meet them outside the airport grounds: a five-to-ten-minute walk with your suitcases, which is nothing at midday and a real pain at two in the morning with children.
It isn't about the quality of the service, it's about who's allowed onto the site. Knowing it before you book saves you twenty minutes of confusion on arrival, after twelve hours on a plane.