Why does NASA (have to?) do things so much more complicated than in KSP?

[karolus10]

Parachute mechanics are broken in KSP, they had very small canopy (radial chute size is enough for one kerbal, but not not for pod) compared to deployed drag and their casing and IRL parachute won't stop You instantly once deployed.

Even on Duna, parachute aren't enough to stop You completely.

[nettcod]

This explains it pretty well.

[Ralathon]

Also, note that our mass ratios are skewed rather unfavorably for us.

Parts in KSP are HEAVY. Really heavy compared to the stuff you use IRL. For example, the ascent module for Apollo weighed about 4.5 tons. 2.3 tons of that was fuel, so that means we have 2.2 tons left for the command pod, engine, RCS etc.

The 2 man lander can alone weighs 2.5 tons, and the LV-909 weighs another .5 tons. In effect this means that staging in real life is much more efficient than it is in KSP. When you stage this means your lower stage needs to lift the upper stages, the lighter the upper stages the better, so you don't want bloody heavy engines eating your TWR.

This is why it is cheaper for us to do a single stage lander while IRL you'd use multiple. It is a necessary evil I suppose, else getting into orbit would be ridiculously easy. We either have huge planets that make getting into orbit take so long it becomes boring or we have heavy parts that discourage staging.