Rocket engines struggle to throttle down to low levels due to combustion instability, injector dynamics, and turbopump limitations.
Here are some stats on minimum throttle levels:
SpaceX Merlin 1D: ~40%
Rocketdyne F-1 (Saturn V): ~70%
Space Shuttle Main Engine (RS-25): ~67%
Blue Origin BE-4: ~20–25%
Falcon 9 does the "hover slam" where they have to turn off the engine exactly at touch down, or the rocket starts to go back up again. Throttle is too high for the weight of the booster at that point in flight.
SpaceX Merlin 1D: ~40% Rocketdyne F-1 (Saturn V): ~70% Space Shuttle Main Engine (RS-25): ~67% Blue Origin BE-4: ~20–25%
Falcon 9 does the "hover slam" where they have to turn off the engine exactly at touch down, or the rocket starts to go back up again. Throttle is too high for the weight of the booster at that point in flight.