Why the popular electric vehicle maker has been struggling.Rivian (RIVN -2.25%) turned in mixed first-quarter results. The electric vehicles (EV) maker's vehicles continue to be popular, but the company continues to sell them below the cost of making them. Given this, the company not surprisingly is burning a lot of cash as it loses money on every vehicle it sells.Let's take a look at Rivian's most recent quarterly results, what the company is doing to improve its operational performance, and its current ca ...