Tesla's core business of car manufacturing is extremely profitable. Their automotive gross margin is substantially higher than most of their competitors, including the likes of Mercedes and BMW. It's only after you add on the costs of growth (capital expenditure, R&D, etc) that overall company profits become marginal.

This is because Tesla is doing exactly what you'd expect from a company undergoing rapid growth: they're spending every dollar they can on growing. Yes, Tesla have received a windfall of cash from credit sales, but all these have done is turbocharged their growth spending. If they didn't have this extra cash, they probably would have had to wait 12 months before beginning construction in either Berlin or Texas.