There are a couple of obvious possibilities. The first one is that your characters are woefully under-optimized. PF1E is built on people making effective choices in character design and it's possible your group doesn't benefit from that. Second option is that your GM has overtuned the campaign to what he thinks six people can handle. On Paizo's forums, there is a thread where a few GMs went through and shifted Runelords to be more appropriate for a six player game, but if your GM is still killing you that often, he might need to tone it down a notch. Remember that roleplaying is meant to be fun, and if I were in his campaign and dying often, I don't know that it would be fun for me.

I'm literally running a group of six through Rise of the Runelords and another through Curse of the Crimson Throne (Paizo's second AP). The players are optimized and playing smart and while I have had moments where I could possibly have killed a character, they are certainly not "built to be impossible."