Why do street drug producers lace their drugs?

It seems like street drugs just keep getting stronger, more complex and more dangerous. 

Long gone are the naive old days when drug users were taking more or less pure cocaine, meth or heroin. In more recent years, a whole cornucopia of new drugs have surfaced, under a bewildering panoply of different street names like bath salts, bloom, molly, flakka, krokodil, chalk, etc. 

But, perhaps even more ominously, the drugs available on the street are increasingly being cut or mixed with other chemicals, first with the more potent drug fentanyl, and then with the much more potent carfentanyl. More recently, veterinary tranquilizer drugs like xylazine and medetomidine are being incorporated. All of these have made drug-taking a much more perilous and uncertain undertaking, and the risks of death have escalated precipitously as a result. (Naloxone has made a difference as an emergency overdose treatment for fentanyl, but it has no effect on chemicals like xylazine and medetomidine.) 

Generally, there is no way for users to know that their street drugs are laced with fentanyl or xylazine or medetomidine, or even a combination of all three, as apparently happens.

My question, and the original impetus for my writing this post, is: why? Why would drug producers and supplier go to the extra trouble and risk of obtaining and adding in these dangerous supplements? The only possible reason I have come across is the belief of some experts that adding these sedatives may help prolong the opioid high, so that users think they are getting some "good shit". 

It's hard to get into the heads of drug producers and distributors, but it hardly seems like good marketing to sell a product that kills a good proportion of their customer base. Their customer retention figures are going to take a major hit, you would think. But no-one ever said that these people are good businessmen, or even that they are thinking straight at all (given that they probably sample their own wares on a regular basis).

It's a mystery to me.