First, not all mortgages have fixed rates. Second, for those that do, the rationale is that mortgages (unlike student loans) are secured by collateral. In other words, if you default on the mortgage, the creditor gets the real estate against the equity of which you have borrowed. That makes a mortgage less risky, from the creditor's point of view, than a student loan.
But I feel your anxiety. I am also kind of worried about the prospect of rising rates. Especially since I'll be borrowing about $87,000 over the course of three years!
I didn't mean all mortgages, but I know you can select higher but fixed rates. I don't think its to do with the collateral though, that's visible in the interest rate, I think the reason mortgage lenders do this is to gamble that they can make more money while interest rates are lower than your fixed rate. I think its like health or fire insurance, they take a calculated gamble that the *&^% won't hit the fan and they'll still make money.
I just read on Midjeep's post about a website, that essentially we only have to risk it until graduation then we can consolidate and get a fixed rate. It sounds like with oil prices where they are, the rates won't be skyrocketing in the near future. that's an oversimplification but last time rates were high was when Alan Greenspan tried to put the brakes on the economy back in 2000 to cool off the economy to prevent inflation. With oil high, the economy will be (allegedly) automatically restrained and rate hikes will be unneccesary in my understanding. (granted stag-flation and high interest rates of the 70's were a direct result of super high oil prices but I think that was an anomaly.)