> And why is grit such a good indicator of successful founders?

Based on what? Biographical accounts by successful founders?

Nassim Taleb's Fooled By Randomness [1] covers the topic of mis-attribution of some causal factor X (i.e. grit) to some phenomena (i.e. business success) that can be effectively explained solely by randomness. In the specific case of successfully starting a business, causal factors are often mis-attributed post-facto through a lens that blatantly ignores survivorship bias [2].

[1] https://en.wikipedia.org/wiki/Fooled_by_Randomness

[2] https://en.wikipedia.org/wiki/Survivorship_bias