No one can survive 5 hits if they aren't wearing any armor, that's true, and ingame no one actually does survive that. But think of an armored guys, first there comes thick layer of leather, then the chainmail (which is made of the same material as your arrowheads), then some more leather before the arrow hits the flesh. IF it even makes it to the flesh. Direct hit probably would penetrate the armor, when shot from short distance, but when shot from afar and possibly on rainy weather, the arrow simply loses most of it's power. And this was only medium armor.
On plate armor there is a high chance that the arrow simply bounces off, dealing no damage at all. And even if a direct hit was able to pierce the plate, it would stop there. So that the human inside the tincan remains mostly unharmed. So our knight here (inside the tincan) would probably just say "lol" at your 5 shots from behind his helmet and keep walking towards you. (armor like that weights a LOT, so running was almost impossible, also falling down meant death since getting back up wasn't much easier than running).
Of course, I'm trying to think the situation in reality, where the iron is mined by hand in extremely poor conditions. So getting a plated armor like that meant you were extremely rich.
Ingame however, I think that the archers are balanced quite well. With the shooting rate they have you can make those 5 hits in the time you'd shoot and reload a sniper crossbow once. Also, if you are focusing your attention on the tincans, you're doing it wrong! Don't waste your arrows on them, they deal just as much damage as a light armored ninja does, so shoot the ninja instead!
If you feel like shooting tincans, get a crossbow. Normal bow deals cut damage, while xbows pierce.
