If most heart attacks occur before blockages hit 70%, why do they wait until 70% to stent blockages?

You misunderstand: Blockage doesn't follow a predictable, gradual course of 60%, then 70%, then 80% etc. A 70% blockage may be present for many years and never cause a heart attack. Ever. Meanwhile a 30% blockage can ulcerate, rupture, or fissure promoting a clot to form and suddenly become 90%, 99% or 100% in minutes to hours. Stenting doesn't cure blockages. It only treats the symptoms caused by severe blockage.