It's because slowing down from 26000 km/h to something that wouldn't cause extreme heating (say, 1000km/h) would only be possible by firing thrusters in the opposite direction. Otherwise, any contact with a medium that could slow you down would lead to the same extreme heating.
And of course, the thrusters you'd need would add huge complexity for the shape, and need extra fuel in the stage 2 itself, greatly reducing its cargo capacity.
And of course, the thrusters you'd need would add huge complexity for the shape, and need extra fuel in the stage 2 itself, greatly reducing its cargo capacity.