Can someone explain why the belly flop is necessary. Couldn't the slowing down by air resistance be done using the engine thrust in a vertical position instead ?
Every kilogram matters, because to lift that kilogram you need even more kilograms of fuel, but to lift that fuel you also need some more fuel! It's brutal.
So you save and you cut and you leave out everything that isn't absolutely necessary. Air resistance is almost free. You do need control surfaces, but they're small and hollow flaps. Fuel is heavy.
That's why 100% of spaceships use aerobraking during reentry. It's much, much cheaper than any kind of sci-fi powered descent.
It turns out that even doing the flip slightly higher, already has a massive impact in terms of energy required.
Slowing down from Orbital speed is an insane amount of energy that has to be removed. You really only want the last tiny bit to be removed by rocket thrusting.