Because the dosages are all relative. If "Drug A" is 100 times more potent than "Drug B", but is then cut 100 times over with inert binders and fillers, you're right back where you started from aren't you. Fentanyl analogs are attractive because, among other reasons, they're compact and hence, easieer to smuggle. One gram of Heroin could last me about a day with a habit; one gram of Carfentanyl is enough to kill off a city block.
When you read headlines like "Drug 1,000,000,000 times more powerful than Heroin!" and so forth, it's mostly just sensationalism. In the end, it's the same type of drug, an Opioid. It will get you more or less the same high, although less material is required, that's the only difference.
I'm going to close this as I don't really see it going anywhere.