Interesting. I think the inability to understand here is your inability to understand that a reused gasket is more likely to leak.Because in most cases I've seen, they don't leak when you re-use them.
I can't see why that would be hard to understand.
Not leaking 100% every time. But certainly more likely. So to guard against that higher likelihood, the suggestion is that you replace the gasket every time.
EZPZ.