With some of the high casualty battles, I wonder why there isn't just hundreds of thousands of artifacts at these areas. I am not talking about finding a pot or old furnace at an excavation site; I mean THOUSANDS of soldiers died in these battles. Why is there relatively little proof?
Bodies, at least those belonging to the winner's side, would have been buried. In case of the battle at Lake Trasimene, Livy explicitly mentions:
[Hannibal] then gave orders for the bodies of his own men to be picked out from the heaps of slain and buried. (22.7)
Furthermore, any valuable items would be looted from the battlefield and useful equipment would be taken for re-use (in fact, at the battle of Cannae Hannibal's Libyans were already using Roman equipment scavenged at earlier battles*). Livy mentions that after the battle of Cannae most of the next day had been spent collecting the spoils (22.52).
This is not to say that nothing at all would remain from such large scale battles. Things of low value or broken items would remain on the field. Some of that may have decayed or displaced by natural processes over the years, but some of it still can be found. At the site of Teutoburg Forest quite some low value stuff has been found:
The archaeologists found remains of Roman swords and daggers, parts of javelins and spears, arrowheads, slingstones, fragments of helmets, nails of soldiers' sandals, belts, hooks of chain mail and fragments of armor plate. source
* Gregory Daly, Cannae: The Experience of Battle in the Second Punic War (2002)
(edit: spelling)
Archeologically, pre-modern battlefields leave surprisingly little trace. Bodies were stripped and looted, armour was carried away by victorious soldiers, and then locals or camp followers would pick the battle clean of anything of any value. In most societies, the value of "stuff" was high- something like metal clasps or buttons would be worth keeping, even broken weapons might be scavenged. Once the bodies are gone and any organic material like clothing or leather has rotten away, there's very little left.
The battle of Bosworth, one of the most important battles in English history, was wrongly identified for years. It was actually fought a couple of miles from where most people thought it was. http://news.bbc.co.uk/1/hi/england/leicestershire/8523386.stm