Startups have this weird psychological purity testing I have never seen outside of religious groups.
Effective organizations understand you actually don’t need to look inside the box. If someone is continuing to do good work for you it’s working. You don’t need to second guess their reasons why, or give them a reason to question their commitment.
It’s a basic lack of empathy. Many founders do have the privilege of choosing opportunities and seemingly cant or are unwilling to relate to people just working with integrity and sometimes being fulfilled by their choice of trade, without needing to be married to someone else’s idea and cause.
Also most founders ideas aren't unique and there is often a tone deafness there. Employees with experience have already seen their idea even if it was in a prior cycle.
"Believing in the mission" requires a suspension of disbelief that can reduce the impact of various factors that would otherwise decrease morale (e.g. lower cash compensation, fewer benefits, unfair working conditions, longer hours, etc).
There's a reason many startups are built on hordes of college kids and it's not that they "have more energy" or "are more willing to think outside the box". They're less experienced and thus easier to manipulate. They're also less likely to have dependents they need to take into consideration, don't understand their limits or trade-offs between short-term performance and long-term endurance (e.g. burnout), and are more likely to be naive about their place in the company and the effect/function of the company. Plus, of course, they're "less risk-averse" which is another way of saying bad at judging the odds of certain outcomes.