If the airbags only go off if you are travelling over a certain speed, i wouldn't want to be hit head on by another car whilst i was driving below that speed then!!!!
Why? They are engineered very carefully to only deploy when they would actually have any benefit. At low speeds, providing your seat and steering wheel are correctly adjusted (at least a 10" gap between the wheel and your chest), and you are wearing a correctly fitted seatbelt (ie, nice and tight with no slack, and not wearing any bulky clothing) then the seatbelt will do a perfect job of restraining you.
Modern airbags form just one part of a more detailed "Safety Restraint System" (you sometimes see the initials "SRS" on airbags), which also include curtain and side airbags, seatbelt "grabbers", and seatbelt force limiters.
I'll just have to drive really fast all the time
Like it. Another one to add to my little book of excuses - "officer, I was going too fast because . . . . "