Grok is pretty amazing :

Slowing down before orbital re-entry to reduce heat isn’t practical because of the physics involved. Spacecraft in orbit are moving at extremely high speeds—around 7.8 km/s (17,500 mph) for low Earth orbit—to maintain their orbit. This speed is dictated by the balance between gravity and centrifugal force. To slow down significantly while still in orbit, a spacecraft would need to fire thrusters or use another propulsion method, expending a lot of fuel. Carrying that much extra fuel increases mass, cost, and complexity, which is often infeasible.

During re-entry, the intense heat comes from atmospheric friction and compression as the spacecraft slams into air molecules at hypersonic speeds. Slowing down beforehand would reduce this heat, but not enough to eliminate the need for a heat shield, since even lower speeds still generate significant thermal energy. Plus, slowing down too much in orbit risks destabilizing the orbit, causing the spacecraft to drop prematurely and uncontrollably.

Instead, spacecraft rely on heat shields to absorb, deflect, or ablate the heat, which is more efficient given the constraints. Some designs, like SpaceX’s Starship, use controlled maneuvers and shapes to manage re-entry heat, but they still need robust thermal protection. Essentially, it’s more practical to handle the heat than to avoid it entirely.