Apple probably has their own vertically integrated self-driving / electric car supply chain and their own self-driving / electric car product in the works. Why would the famously secretive company expose their future products before they're ready? Also, as good as Musk is at his work, Cook probably didn't want merge with volatile Musk.

Sounds plausible. On the other hand, Tim Cook went to work with the infamously volatile Steve Jobs, at a time when Cook's employer Compaq was already an industry leader and Apple was bordering on bankrupt.

Yeah, that's a good point. It would be interesting to look at that point in his personal history and determine what drove him. Hmm..

Tesla wasn't worth the asking price then, and wouldn't be worth it now if Apple had bought it.

So both Apple and Tesla dodged a bullet.

Why not take the call and answer "no" ?

Because then you lose some bargaining power if you change your mind down the road.