Yes, Arafat was a known quantity to the US and Israel, but I think you've drawn the wrong conclusion. The fact was that Israel and the US viewed Arafat as a road block to a grand bargain. Israel made their best offer in Camp David, and Arafat refused it. I won't repeat myself (my comment above sets out my view), but if you understand the political circumstances at the time, Arafat was in fact the last remaining major road block to a grand bargain that Sharon was moving heaven and earth to get done.