You pointed to the answer in your question: It's about sustainability.

You work hard at it because you're genuinely trying to build something that will last vs the kind of crap you alluded to that will come and go as long as there is money to be made.

To say it more specifically, you're not busting your hump PRIMARILY for money.

If over 95% of startups fail then the odds are stacked against you before you even begin. On top of that, if money is your primary motivation the odds of you sustaining your interest in this startup are almost guaranteed to fizzle away.