This reminds me of another thing now that you mention - the weird expectations of "rationally defend." The Greek philosophers were unusual in that they had a belief that humans are "rational" animals and that is what is unique about humans. This belief made its way into contemporary Western culture in spite of its weirdness. It is treated like an incontrovertible religious truth by some humans - something must be "rational" or else. You must rationalize yourself or else. That's weird considering the vast majority of human experience and behavior is non-rational. Overthinking and overanalyzing is weird and silly.
Why isn't it enough for someone to just explain where they are coming from and that to be good enough?
Depends on the issue, but I think a lot of this comes down to the "reasonable man" test.
Especially when we're talking about issues that involve legal rights and freedoms, democratic societies generally have the idea that limitations on rights depend on the consent of the governed, but the need for consent has limits: if your argument for limiting someone's rights or changing the law in some way would convince a "reasonable" person, then you've met your burden and you're justified in pursuing the change.
... and typically, the test used to determine what is reasonable is rationality: if your opponent's argument is irrational, then this means they've made a mistake of logic... and relying on an argument we know is mistaken would be unreasonable.
That's my take on it, anyway.