Good question. I think a lot of the experts have him ranked so high based on a) he's the indisputable RB in SF with Hearst gone, and B) his 5.1 yds/carry avg.
He's got the talent to be a top 10 back, but like you, I agree that he's got a terrible offense that will struggle this year, and I don't see him having the year others are predicting.
For those who own him, the good news is he's sure to get the ball often.