If the interest rate goes down, you can just refinance. The fixed rate protects you from upward movement, and being able to refinance protects you from downward movement. What am I missing?
There are fees and process time to refinancing that acts as a disincentive to do so; and for several decades the fixed rate (by design) has been higher than what the variable rate incurs over the term (at least in Canada, where the term and the amortization period aren't the same).
Also: variable mortgages can also convert to fixed. If the rate is sitting flat, you're better off variable, with the option to switch to fixed if it looks like it will climb, than vice versa.