You're a little young for it and it probably hasn't been taught yet, but I'll give you the gist.
Everything has a sort of hardness/firmness to it. There's a rating scale of 1-10, 1 being the softest and 10 being the hardest, it's called the Mohs scale of hardness. 1 would be say, talc (baby powder), and 10 is a diamond.
In this scale, things can only be damaged if it's the same number or higher. A 7 can scratch a 3, but a 3 can't scratch a 5, etc.
Plastic is a 2-4 on the scale depending on what it's made of, and sand is a 7. Sand can go around in the air as dust, so here you have particles of hardness 7 floating around. They can land on your Tamagotchi and then you wipe it with your hand, you're actually dragging those sand particles across the surface and scratching it up.
But yeah, it will happen. best bet is to protect what you can, such as the screen--that's more important. Get a screen protector if you can.