Tesla had deals with Mercedes and Toyota to supply drivetrains, batteries and co-develop EVs a long time ago.... maybe if we understood why those deals withered on the vine, we'd understand why OEMs aren't beating down Tesla's door for similar deals today?
Here's a hint at one of the factors: Mercedes and Toyota both recalled their co-developed vehicles for issues with the Tesla powertrain. Tesla-branded vehicles had similar issues with sudden loss of power, but there was never a recall. newcartographer.com/combustion/tes… 
The factor hinted at above is not that Tesla's stuff was shoddy (though there is reason to believe it may have been) but that the culture gap was too wide to bridge. Auto OEMs rely on culture to catch issues top managers can't always see, and partners have to share core values.
I wouldn't even say this culture clash was all Tesla's fault. It takes two to tango, and automakers have plenty of arrogance and "not invented here" in their DNA. But they went into the relationship wanting to learn, and what they learned didn't keep them around.
Investors love Tesla's arrogance, but making cars is a relationship business and Tesla's record on that front just isn't great. It's not just OEM partners, Tesla's supplier relations have tended to sour over time too. In this business people notice that pattern, and stay away.
You'll notice that "vertical integration" has become this Tesla buzzword, without a single instance of anyone ever substantively demonstrating the economic advantage of insourcing. This reinforces my view that it's just a way to spin poor supplier relations into a positive.
Nobody actually believes that Tesla is better or more efficient at making seats than Lear or Faurecia. It's an absurd notion on its face. But you can't tell them "deliver 5k units/week of parts" in Summer '17 and then spend >a year actually getting to that level. That doesn't fly
Ultimately, Tesla always puts its short-term image first. Whether that's avoiding a negative news cycle with a stealth recall, blaming a problem on a supplier, or ditching a production schedule because they can't keep up with Musk's hype. That's a recipe for failed relationships.
