How did 70s/80s spoof movies get this density of gags?

I grew up with Airplane and Spaceballs and Hot Shots! and the amount of both visual gags and puns is still astounding. Some you only get as an adult, some you'll never get because you're missing a frame of reference (for example "he never drinks a second cup at home").

Currently there is a news headline going around where people suggest that Brazil should introduce a O2 fee for the upkeep of the Amazon rainforest and immediately people jumped in with pics from Spaceballs and President Scrooge taking a nose from the Perri-Air can.

And I just began to wonder how you can create a Star Wars spoof movie and then just throw in a Perrier sparkling water reference. How many rewrites and add-ons go these scripts through and how many people throw in a few more puns during writing and filming.