First of all, because by doing this you’ll get a lot of egg white on the inside and outside edges of your bowl that you’ll have to pick up/clean afterwards.
Secondly, you have to be careful with the hygiene of the eggs, they come out of a hen’s ass (no scoop) and unfortunately a hen’s intestine can contain salmonella, bacteria that are sometimes present on the outside of the shell and can pass from your bowl into your recipe, and risk making you sick.
Well, you shouldn’t be paranoid about this either, the shells of eggs from the agro-industry are washed before they land on the shelf, you just have to be a little more careful with eggs from your henhouse, or those bought at the market directly from the small producer.
And then if your recipe goes into the oven afterwards, no more risks, but for a mayonnaise for example it’s trickier.
All this to say that it is better, if possible, to avoid contact of the eggshell with your preparation in progress, and that therefore this way of splitting the eggs on the edge of the salad bowl is not great.
How do you do it then?
It’s very simple, tap the egg on the surface of your work surface, a short sharp tap to start breaking the shell.
Bring the pre-cracked egg to the top of your bowl, and finish breaking the shell by dipping your thumbs inside, and pulling it apart to let the egg fall into the bowl.
Throw away the shell, move on to the next egg.
With this way you are sure to make, no shell/bowl/preparation contact.
To sum up: To break eggs into a preparation, don’t knock on the edge of the bowl, but rather on the work surface.