Test kits are available, lab testing is available in some places, most drugs are easy to "clean up", and some sources, while shady, rely wholly on reputation.

There's also a risk/reward factor - many drugs are incredibly enjoyable, and users feel it's worth the risk.

Oh, I've no doubt participated, it's just crazy because who knows what's actually in it.

E.g., fentanyl.

... Thats where the mentioned test kits are for :)

Yes because generally people who sell shit want to keep selling shit to the same people. So there is an incentive to sell stuff that is as close as possible to what the customer wants. Yes you get cases where people cut coke with fentanyl, but in general this is not how it works.

drug users don't care otherwise they wouldn't be drug users. Look up Krokodil, addicts don't care what it does to them as long as there's a high somewhere in the effects. It basically impossible to stop demand side which is why the (futile) efforts are spent on the supply side.