Consider the alternative of burning less than 1G of acceleration, you'll fail to slow down although you'll descent acceleration will be somewhat lower. At exactly 1 G it'll keep falling at a constant speed till it runs out of fuel, then impact.
Or if you're asking why performance is so much more spritely at the end of mission, unlike boats or cars the fuel in the tanks is like 95% of the mass of the rocket, so something that barely waddles off the launch pad with full tanks weighs practically nothing at landing when the tanks are nearly empty.
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.