Reasoning is based on knowledge. There have to be things you accept as truths first before you can start reasoning, and those truths are not universally shared, nor do they have the same weight for everyone. That includes you and me.
There are things we don't "know", and things we don't know that we don't know, but we nevertheless think of ourselves as informed and capable of reasoning. To someone who knows more than us, they'd consider us stupid. It's not about objectivity, it's about looking down on those that don't know things you know and declaring them less-than.
The basic point is there are countless factors big and small that influence any individual's thoughts and ideas at any given moment. Our minds are very complex things, and our lives are messy, absorbing all kinds of information and stimuli that affect it in ways we don't properly understand or even realize.
When we talk about people being stupid or smart, we're just reducing that complexity so we can make simplistic insults that make us feel better about ourselves, but ultimately aren't saying anything meaningful about the human condition.
And there's a lot of dark history behind this, too. The history of psychology is riffe with falsehoods about quantifying intelligence, and often it was simply about prejudice.
You want to call people stupid for doing stupid things, sure, I get that. I do that. We all do. But the more you try to create these general arguments about human stupidity, the more it unravels, and the more it reveals about you.