Sometimes, the best way to design a mission is to work backwards. In this case, start with the return capsule and design it to meet the requirements of reentry and landing. Once you have that, the proceed to work the mission in reverse until you have it ready for the launch pad. What you're likely to find is that for every addition kilogram you add to the reentry capsule at touchdown, you may well end up needing hundreds of additional kilograms of mass (booster, propellant, etc) at the launch pad to fly the mission. It's a gigantic snowball effect on mass and cost.

If they'd added airbags to the landing capsule, then not only would it have added to the capsule cost but likely the capsule's mass. The alternative would've been to reduce the mass some other way, perhaps by going with a more expensive construction material, cutting some reserve margin, or by reducing the payload. Nothing is free. Plus, the more things you depend upon for a safe landing, the more likely it is that one of them will fail and jeapordize the landing altogether. "Simplificate and build in lightness" is sound advice for designing airplanes. It's absolutely crucial for designing spacecraft.