Because we have hindsight, we know what the person would say and how they would react.

i think because we may need time to think things over ?

Because there's more time to think. Like when you're arguing with someone and later, in the shower, you're thinking about what you could have said

Bc you've got more time to think about your answer. I think.

Because hindsight is faster than our mouth. Always one step behind.

bcs only after it ends we truly stop to think about it