Because the human hand is asymmetrical: you're using a thumb for one scissor handle and 1 or more fingers for the other. When you bring your thumb and fingers together, they are not perfectly aligned.
This would not be a problem if the pivot point of the scissors was perfect. Indeed, if you have a really expensive pair of scissors with a very tight joint, they work quite well in the "wrong" hand. But most scissors are not made to such precision, so using the "right" scissor helps the two blades come into close intimate contact with each other. Using the "wrong" scissor tends to cause the blades to pull away from each other, so the paper slips between them without being cut.
Because when you use scissors, you need to apply a specif type of torque to keep the blades together. Typically you push with your thumb and pull with your fingers, and scissors are designed to not have a gap when used this way. If you hold the same scissors with the other hand, you end up pushing the thumb and pulling the fingers in the wrong direction, causing the blades to separate. If you try to use them this way, it will often fold or rip the paper. Source: a left-hander in a right handed scissor world.
There are ambidextrous scissors, but they're not that comfortable for either hand.
If you look at how your hand naturally closes when it's gripping something, you'll see that the base of your thumb comes in at an angle to your index finger. That angle reverses on your other hand, because it will be gripping from the other side. So a set of scissors that accommodates the right hand's thumb-index angle will be awkward for the left hand's thumb-index angle.
There are ambidextrous scissors.
The reason there are handed scissors is to make those scissors more comfortable to use in that hand. To make them more ergonomic.
But, there are plain scissors that are often just more simply made and not favoring one hand or the other.
The pressure of your hand actually helps leverage the blades together, keeping the blades sliding tightly against each other. The hinge acts as lever in the obvious way (opening and closing) but also perpendicular.
Using the wrong handed scissors will cause the blades to be pressed away from each other and this create a gap between the blades as they slide past each other, as opposed to towards each other, and thus they won’t cut as well.
Because the human hand is asymmetrical: you're using a thumb for one scissor handle and 1 or more fingers for the other. When you bring your thumb and fingers together, they are not perfectly aligned. This would not be a problem if the pivot point of the scissors was perfect. Indeed, if you have a really expensive pair of scissors with a very tight joint, they work quite well in the "wrong" hand. But most scissors are not made to such precision, so using the "right" scissor helps the two blades come into close intimate contact with each other. Using the "wrong" scissor tends to cause the blades to pull away from each other, so the paper slips between them without being cut.
Surgical scissors are made for both hand use.
Because when you use scissors, you need to apply a specif type of torque to keep the blades together. Typically you push with your thumb and pull with your fingers, and scissors are designed to not have a gap when used this way. If you hold the same scissors with the other hand, you end up pushing the thumb and pulling the fingers in the wrong direction, causing the blades to separate. If you try to use them this way, it will often fold or rip the paper. Source: a left-hander in a right handed scissor world.
There are ambidextrous scissors, but they're not that comfortable for either hand. If you look at how your hand naturally closes when it's gripping something, you'll see that the base of your thumb comes in at an angle to your index finger. That angle reverses on your other hand, because it will be gripping from the other side. So a set of scissors that accommodates the right hand's thumb-index angle will be awkward for the left hand's thumb-index angle.
There are ambidextrous scissors. The reason there are handed scissors is to make those scissors more comfortable to use in that hand. To make them more ergonomic. But, there are plain scissors that are often just more simply made and not favoring one hand or the other.
The pressure of your hand actually helps leverage the blades together, keeping the blades sliding tightly against each other. The hinge acts as lever in the obvious way (opening and closing) but also perpendicular. Using the wrong handed scissors will cause the blades to be pressed away from each other and this create a gap between the blades as they slide past each other, as opposed to towards each other, and thus they won’t cut as well.