Well a lot of them are being found all the time. For example here is a find from Scotland a couple months ago, where the construction of a couple of football (soccer) pitches uncovered a 3,000 year old sword, spear, and related material. Then there was this sword that was found last weekend. Such finds are sufficiently common that only ones that are particularly noteworthy for how old or how well preserved they are are likely to show up in the news.

Of course with most metallic objects, their preservation is dependent on local conditions. Iron rusts, and most other materials have some form of rot or oxidation. Wait long enough and most iron arrowheads are likely to end up a smear of iron oxide in the ground. Most bows will turn into mulch given enough time.

That said I do think you are underestimating the old philosophy of reuse - reduce - recycle. A lot of the oldest Christian churches are made out of stones taken from pagan temples. Why quarry new stone, when perfectly good dressed stone was sitting right there? Metal was fairly valuable and was worth scavenging. Not every weapon would be found after a battle, but it's unlikely that the locals wouldn't loot anything that wasn't nailed down.