Why do some puppies bite and try to eat everything they see?

Because they are puppies, and they are curious. And they are just getting puppy teeth.

puppies mouths are like out hands

They are learning about the world around them, very much like human babies put everything in their mouths...

Puppies and babies explore the world by putting everything in their mouth.  Until they learn to not bit so hard /not grab so hard, puppies and babies can hurt.  It's all part of growing up.  (You did it too when you were a baby.) 

Because many owners are clueless.

They're teething. 

All puppies do. It's not just some puppies. It's just like how all babies do it, except with puppies they have to do it because if they don't their jaws and teeth deform. You can train a puppy to not do it to everything they see but only specific things like chew toys, but that takes time, and until then all puppies will do it until, like humans, most of them grow out of it, although, like humans, some don't for whatever reason, the theory for humans not being generally some trauma, so probably the same for puppies that don't, and so you have to work a bit harder to get them over it, just like with humans that get stuck in what's called the oral phase of development.