Picky eating.

Why is it that babies will refuse the meal you spent ages preparing, and other offerings of fruit and such, but eat anything off the floor?
I am really struggling with keeping things out of his mouth. He loves plastic, paper, and cardboard. Most of his books look like they have been attacked by rats. I do give him lots to eat. Both breastmilk and meals, so I know he isn't hungry. He is a good eater of regular food (except the last few days due to teething and having a cold), so in that I am lucky. I just wish he would stop eating every speck of food off the floor.
Thank goodness we don't have pets with pet food lying around or this would totally be Pichan...