And then it was downvoted, and I was very curious to the difficulty of posting spoiler tags, or why do people feel so adamant in not using them.

Jesus fuck... Just how tagging spoilers is obvious to you and me, the fact that downvote button is "I disagree and don't like you" button should be obvious to any not braindead user.
You know why people downvote objectively perfectly reasonable posts? Because those people are too stupid to understand them. It's an universal rule on reddit.

why do people feel so adamant in not using [spoiler tags]. It's not like it costs them anything.

Well, I get your point, but I'm actually against spoiler tags being used for every single minor thing. It reeks of "trigger warnings" being almost forced on people. I get that a heavy spoiler can worsen your enjoyment of a particular work of fiction, but there should be some leeway for minor, insignificant spoilers. I'm going to use an example from my life - my friend spoiled to me a particular plot twist in Blood Omen 2. I was kinda pissed, because not only it was an event late in game, but was also quite important to protagonist's character development. That's the kind of spoiler one should tag.
But untagged minor spoilers, like stuff you get to know minutes into the game or comic series, or stuff that's ancient, well known, and has reached a memetic status, like mentioned "Vader is Luke's father" or "L dies", shouldn't be punished with censorship/bans. So, despite being s-f fan, Luke's lineage was a surprise to you? Under which fucking rock have you lived for past three decades? Have people became so think-skinned, overreacting little faggots that they can't overlook minor spoilers?