singular vs plural - Why does it have to be 'scissors' and not just 'scissor'?

I'm wondering why I need to say scissors instead of just 'scissor'. Are there any rules about it?

I would think that I only have one scissor, meaning it would be singular and not plural. (That's at least how it is in my native language.)

This would only be one scissor in Danish.

Can anyone tell me why it needs to be plural?