^^ actually pein they did! Try looking closely those wings.
They did evolve! Chad nailed the answer about living-fossils!
One example is that of the elephants. They are an evolved Mammoth. There were giant sloths too! There are tree climbing sloths and there are earth walking sloths. Species have different groups although they are in the same family. Reptiles, cats, dogs, tigers, fishes... they evolved.
As we have explained that there are several branches of specie... they may stay behind on the process and some grew much larger because of the food they eat.
The reason why they are fossils now because they died/trapped from a sap of a tree or frozen. If they hadn't been in that fate... we might have giant dragonflies. Kuha na nimo pein?