It took NASA...four years to design a drone.

So why not contract a drone mfg instead of (likely) spending $20 for every $1 the experienced manufacturer would? Isn't that what Space-X is all about?

SpaceX is doing a decent job making established technology (in some cases decades old) more available than previously.

There is nothing in what the company has done to date that suggests Space X could more quickly or at less expense design a drone that has any better chance to operate in an environment which we on Earth have limited knowledge than NASA.