Musk has also said that growth in his companies are mostly people constrained and not capital constrained. His companies can raise capital any time they decide it is a good idea.
But leaving recent history aside: the fact of the matter is that Tesla is now valued at 136% the market capitalization of Toyota, an organization that currently ships about 50x more vehicles each year than Tesla does.
The only possible way for Tesla to achieve earnings from car sales that justify this stock price* (on a reasonable time scale) is to build out their manufacturing capabilities at an unimaginable scale and pace. The amount of capital that this will require is vastly higher than anything Tesla has raised before. And since the period we're discussing is long enough that we can't predict global market conditions, Tesla may find themselves doing this remarkable build-out during a period where capital isn't easy or inexpensive to come by.
* I'm assuming here that the market cap of Tesla reflects some realistic theory about future earnings, and isn't just a massive and unjustified bubble. Which it probably is.
[1] https://www.theverge.com/2019/5/17/18629166/elon-musk-tesla-... [2] https://finance.yahoo.com/news/tesla-may-avert-bankruptcy-bu... [3] https://www.wsj.com/articles/tesla-shares-sink-as-musk-jokes...
The fact of the matter is that the entire purpose of the stock market is supposed to be to direct capital into productive investments. There is absolutely no way that Tesla management can look at the next 10-20 years of global economic and business developments (a period in which Tesla will likely be fighting for its survival as traditional automakers finally begin to compete in the EV space) and conclude that "gosh, there is no set of circumstances in which we could execute better with a few billion dollars of additional reserve capital in the bank."