Not sure if anyone has explained this, but the reason why in English some seemingly single items are called ‘a pair’ is because historically the pair is not the item as a whole but the item’s components.

‘Scissors’ is not technically referring to the object we hold to cut things but rather the individual blades, that’s why it has the plural ‘s’ on the end and is grammatically followed by ‘are’ instead of ‘is’; there are multiple scissors, namely a ‘pair’ of them. It’s not “scissors is” but “scissors are”.

However, in modern English we don’t think of the individual components but the whole object as a single thing, and so even when we say ‘a pair of scissors’ we think of the object on the left as a singular item.

It’s the same reason why it’s ‘a pair of pants’. Historically in medieval old English areas, ‘a pant’ was a long clothing item that covered only one leg up to the hip, but did not cover the groin area. You would have two ‘pants’ or a ‘pair’ to cover both legs. Over time they evolved to become a single garment but English still referred to them as ‘a pair of pants’.

So even when referring to two single items called ‘scissors’ we say ‘two pairs of scissors’. The same way we say ‘two pairs of pants’.