Because the banks are always ahead on fixed rates. Think of it like this:
Say the economy is going down, variable rates are falling. Fixed rates fall to in anticipation of rate decreases in the future. Now, two scenarios can be played out here:
1) Once the economy looks like it is about to recover, the yield curve becomes positively sloped again. This means that fixed rates will go above variable rates, and even if you fix you will be paying a lot more than the variable rate with no guarantee that variables will go above the fixed rates. So banks win.
2) The economy gets worse, and rates fall. They will fall below what you fixed it at. You lose, banks win.
The exact same thing applies if the economy is booming and you want to fix your rates so that you are insulated from rising interest rates. Many, many people got caught up in this back in 2007/08 when they fixed rates at 8-9% only to see them fall down to their current levels of 6% today.
Plus, banks adjust their fixed rates all the time based on the money market and how the yield curve is looking. They will always give you a fixed rate when it suits them. You are betting against a company that makes it living off betting on interest rates - I don't like doing that. Plus you also have the other inconveniences of fixed rates like limited redraw, limited offset, break costs if you refinance/sell, no equity release etc.
I finally agree with one of your posts Aaron. Good post