I didn't say we had to "work" to be saved. You can't work hard enough to be saved, because no amount of work can save you - only Christ can.
As for why God would create us, even though He knew we would sin (I wouldn't say that He created us sinful, just with the capacity for sin) that's a good question, which lots of people differ on. To put it simply, though, I'd say it's because a universe with fallen, but then redeemed man is better than a universe free from sin, but also from God's Children. As for why that is - because there's something inherently good in God creating, or in our lives or our future existence that outweighs the bad, or something else - well, we'll just have to argue back and forth over that.
That's my take, anyway. Anyone else is free to chime in.