It's not attaining anywhere near orbital velocity, but it's still going pretty fast. All that speed has to be shed somehow, and they do that through atmospheric

It's not attaining anywhere near orbital velocity, but it's still going pretty fast. All that speed has to be shed somehow, and they do that through atmospheric drag. The deceleration is high because the atmosphere gets thick pretty quickly, so you end up losing all that speed quickly, which means a high acceleration. They could use the rocket to slow down --- but to do that, they'd have to carry fuel up, which makes the whole thing much larger. It's vastly more efficient to let atmospheric drag do it for you for free.