They have throttle, but not enough throttle range. Merlin is a 845 kN engine that can throttle down to 338 kN. When the rocket is mostly empty, that's too much

They have throttle, but not enough throttle range. Merlin is a 845 kN engine that can throttle down to 338 kN. When the rocket is mostly empty, that's too much thrust to hover. So they have a very short window to start the landing burn, and also not enough fuel to abort and try again. Given the constraints, it's surprising they don't lose more rockets during the landing phase.