says, it's about thrust and ability to throttle.
Atlas V has 1 engine with 2 combustion chambers and relies on solid boosters for anything past that. Not sure how low RD-180 can throttle and I really can't be arsed digging through my books to see if I can find it, but there's no fucking way it can throttle down low enough to slow down let alone hoverslam.
Now the Falcon 9 has 9 engines which can throttle pretty low due to having a very good pintle injector, it uses 3 engines to slow down and protect the engines during reentry (sounds kind of paradoxical, but the fire from the engines actually protect them coming through) and it uses a single engine for the landing burn.
It's a matter of balancing the amount of thrust per engine to the rocket's size. That's why Starship is so xbox hueg. With the amount of engines on the super heavy, you can do the same re-rentry as the F9 booster and the same landing, but not for the starship since it only has 3 sea level Raptors. So it will need to bellyflop and use air drag to slow down.
Another comparison is Vulcan Centaur using 2x BE-4 vs New Glenn. Vulcan will potentially use SMART, which is just engine recovery, New Glenn will be able to theoretically land propulsively.