That's not really the truth there have been numerous studies where people have bought random samples off the street and most of it was at least 50% pure. Cocaine is a nice drug but it's not the "wham bam" smash in the face that a lot of people expect unless IVed or smoked in its freebase form. I think Cocaine is a decent stimulant, but unless you have tons of money to blow and don't care about wasting it you can get plenty of stimulants that are just as good or better for a fraction of the price - a large part of Cocaine's popularity is just its reputation as the "rich mans drug", a lot of people get the added rush just off the fact they're sticking insane amounts of money up their nose in a single weekend.
Most of the other competing stimulants out there are purely NDRIs where Cocaine is an SNDRI which is why it's somewhat subtler (particularly at low doses) and more relaxed until you IV/smoke or push it to high doses, there aren't many SNDRIs available that are decent and that's another selling point for it.
Personally though I've had good coke and it's a waste of money to me, I'd much rather take some a-PVP or even MDPV for a night out than coke, they have more legs (duration per dose), much lower dosage required, less cardiac side effects, the price is nothing in comparison, and I can stay going all night, share with everyone else at the party, and go home with nearly the same amount I took to the party with me in the first place.
Tl;dr Cocaine is a decent enough drug, but the price to high ratio of Cocaine is probably the worst out of any drug on the market, and you can easily emulate and beat the high for a fraction of the price. Some people though as I said just get off on the fact "hey I'm doing Cocaine" and that's enough of a selling point to them to make it worth the price. I'd rather get my money's worth and get a better night and have my pocket full of money and drugs still at the end of the evening though.