While some of the SO practices can feel dumb, I wonder what is the tradeoff we are making here? Could it be that for whatever reason (e.g. personality) we might benefit from accepting these "dumb" choices because it also brings with it unrelated benefits? For example, might it be that if they were forced to accept "frivolous" statements such as "thank you" (from the example you gave), might it cause moderators to not moderate, and thereby allow in spam & other nasty bits?

It is worth bearing in mind that if we, "good people", complain about moderation, we only see the parts of it that touch our "good posting". There might be plenty of good that moderators are also doing, which only the bad guys see.

"I was slighted once by a completely opt in community that helps millions of people every day so it deserves to die"

Why is this everyone's default response to this kind of stuff?

Because it'd be super simple for a mod to say "oh, no problem, you can say thank you." Instead, it's like "NO WAY CAN YOU SAY THANK YOU DO YOU REALLY WANT TO WASTE PEOPLE'S TIME WITH... pleasantries? ::ban hammer noises::"

Between this and the reddit blackout, "volunteer" mods doing free work for a company are really overstating their importance. The cons don't outweigh the goods. It's why I like the Fediverse: I control what I can see on my own instance and don't have a rando telling me what I can and cannot do.

A rando can't deny your freedom to post, but they can deny you interaction with anyone beyond your own instance. Most instances have a pretty long blocklist (albeit sometimes for good reasons).

A single rando can deny your interaction on their instance, not on a bunch of other instances. Shared blocklists notwithstanding, but I would hope if you're following a shared blocklists you're vetting it to an extent.