If there’s things as they are, and things as we see them, why does everyone claim to know exactly how everything works

Consensus of opinion. Subjectivity becomes objectivity via consensus.

Perception isn't always reality, but with sciences we have a way of verifying that things really work.

Some of it is not complicated, even without science. You're driving a car. There's what we see to be a telephone pole. But is that perception? Drive your car into it, and you'll probably find out.

Because some things add up to the same end result, despite our perception. For example. Loss of a loved one is loss, regardless of how individuals cope with the grief of it.

People like to be right. It scares them when someone else has a different perception... Like it screws with their reality so they argue about it and believe they are justified.

I think you answered your own question

A person can have a go to source for the facts.