Why don't pennies and nickels have serrated edges like dimes and quarters?

The ridges on dimes and quarters are from back in the day when they were made of precious metals (usually silver). People would shave off a few fillings from the edges of coins with a file; these shavings would add up to a good amount over time.

The ridges were put there so you could tell if someone had filed down the coin or not. You wouldn't accept money that had been filed down.

Since nickels and pennies weren't made of silver, no one would file them and hence there was no need for the ridges.

Well I am not too sure about modern times, but I do remember in olden times they used copper to make pennies and they used nickle to make nickles and they used to use silver on the dimes and quarters...

And they all came from different molds, and it had something to do with making sure no two coins were alike in size so they would be distinguishable to any one making change...

Who knows...?