Their intentions, although misguided, are for the best to motivate their children into doing better. Criticism actually squashes any feelings children have of trying something new, and results in them feeling alienated from their parents. Criticized children end up feeling angry, worthless, unloved and undeserving, and their self-esteem drops. Around and around it goes. It's a vicious circle that parents keep alive with their continual hurtful criticism.
Young children view their parents as all-knowing, and take in their words as gospel truth.
Children don't filter out the good from the bad but instead, like a hidden tape recorder in their Bo-computer, store it all. These words spoken to a child, form the internal monologue we all have running through our minds as adults. If Mom thinks I'm clumsy, dumb, stupid, an idiot, fat, lazy, etc. Most of all, this is on an unconscious level, yet that doesn't negate its importance at all. Soon he comes to see himself as a sloppy person and inevitably becomes more sloppy.