His investors are not investing because of his success rate in delivering on his promises. His investors are investing exclusively because they believe that stock they buy now will be worth more tomorrow. They all know that's most likely not because Elon delivers anything concrete (because he only does that in what, 20% of cases?), but because Elon rides the hype train harder tomorrow. But they don't care if it's hype or substance, as long as numbers go up.
Elon's investors are happy with his success rate only in terms of continuously generating hype. Which, I have to admit, he's been able to keep up longer now than I ever thought possible.