There's a major problem with the idea that Apple should buy Tesla

It's become predictable. Every time it looks like Tesla is up against failure - as it could be now with its ambitious production targets for its Model 3 mass-market car - calls are renewed for Apple to use some of its massive hoard of cash to buy the $30 billion electric-car maker.

On paper it looks synergistically appealing. Tesla would be freed from its state of perpetual anxiety about its future, and Apple would make a bold move into transportation, now viewed by many in Silicon Valley as the next great disruptive frontier.

Also, there aren't that many companies that could swallow Tesla whole. Apple is one of them.

This all overlooks one huge, critical problem, however, a problem that is pretty much insurmountable: There are no two CEOs on earth who are more different than Elon Musk and Tim Cook.

Cook isn't an entrepreneur

Cook's genius is in supply-chain management: If it weren't for him, Apple wouldn't have its mountain of cash. As Apple has printed money with the iPhone, it has sucked profits out of its suppliers and the wireless providers.

This is the deal, like it or lump it. But Cook isn't a person who brims with mad visions and wants to take wild risks. If he were, Apple would be selling a TV now instead of an overpriced digital watch.

Musk works for no man

When Musk started PayPal, he was part of an oddball genius collective that included Max Levchin and Peter Thiel, both offbeat personalities (Levchin was born in the old USSR and his parents fled, while Thiel is among the world's most prominent and wealthiest libertarians).

These guys still don't work for anybody, and probably never will. Musk presides over two companies, Tesla and SpaceX, as CEO and one, Solar City, as chairman. He personally oversees Tesla design and has made himself into a de facto rocket scientist. In a thousand years, his leadership personality would never dovetail with Cook's.

Cook is starting to fail

The Great Apple Story - which began with the return of Steve Jobs and reached its apotheosis several years ago when Apple became the most valuable and beloved company in the world - is leveling off. As the iPhone has reached full maturity, it's become harder and harder to figure out what Apple does next. The Apple Watch isn't creating an iPhone-like ecosystem of must-have wearable technology; nobody knows what's really going on with "Project Titan," the Apple-car project; the obvious move into TV doesn't look as if it is ever going to happen; music streaming and other "tech" services are boring and unromantic.

No one should be surprised. Cook isn't an idea guy. And Apple's idea people are largely designers, like Jony Ive, who ruled the day under the post-hippie aesthete and visionary Jobs, but who are now living in a world where the action has shifted to the grinding development of huge platforms that can support advertising. Apple is in the beautiful-things business, as is Tesla, and that's one of the arguments that drives the takeover talk. But Cook doesn't want to be in the position of buying Apple and seeing Musk turn into the new Jobs.

Musk needs to keep Tesla independent

Tesla is just one piece of Musk's grand vision, which is to accelerate humanity's escape from a dependence on fossil fuels. Tesla's role is to spur the arrival of widespread electrification in the world's fleet of vehicles. Even though Musk now wants Tesla to be delivering 500,000 vehicles by 2020, the goal isn't for Tesla to be the Toyota of EVs. SpaceX's job is to "back up the biosphere," according to Musk, and make the species multiplanetary - in case the earth's environment is rendered uninhabitable for humans.

Musk's companies serve a larger purpose. But if Apple were to buy Tesla, the acquisition would have to make some contribution to Apple's business, preferably a tangible one. I'm not sure that Apple would be satisfied with having the first Genius Bar on Mars.

Cook is a relatively normal person

No CEO of a major tech company has any hope of living a normal life, but Cook seems to exhibit none of Jobs' eccentricities. He likes fitness and appears to live modestly. He's among the prominent gay executives in business, but his sexuality isn't anything that has provoked controversy. He presents Apple's products cheerfully. He has been an excellent steward of what Jobs and his Apple cofounders created.

Musk is an exceptionally unusual person

There are many reasons why. I'll muster a single example: After he sold PayPal, Musk taught himself rocket design by reading old Russian rocket manuals. Maybe another: On earnings calls with analysts, he loves nothing more than to dig down into the technical minutiae of Tesla's vehicles. He lives the life of a tech celebrity: Robert Downey Jr. modeled Tony Stark from "Iron Man" on Musk; he's been married several times; he has six kids; he splits his time between Silicon Valley and Beverly Hills; he sleeps next to his Tesla assembly lines; he oversees SpaceX launches. He is by all accounts a wonderful father and decent person, although demanding to work for, but he's not spending many quiet evenings at home.

So if Apple were to buy Tesla, someone would have to go, and that someone would be Musk. And Musk doesn't want to go.

That said, there's no shortage of auto-industry veterans who think it's time for Musk to give up the big chair. And in the unlikely event that it happens, there would be one obvious plus: Musk could be all-in on SpaceX's race to put people on Mars.