Overdose is less likely to happen with an unadulterated product because the dosage can actually be measured. You can't measure a dose with a product of unknown concentration. The dose response curves and ED50s are known for all of these substances but people keep dying because they don't actually know how much they're taking

you're less likely to die from a fentanyl shot that you know the exact concentration of, than injecting adulterated white powder with nothing to base the dosage on.