It's very rarely that drugs, including alcohol, feature in public transport aircraft accidents. We get into a cost-benefits argument then, given that this problem does not seem to be a large one. Simply put, there are more beneficial things to spend money on when trying to improve the safety of air travel than increased drug testing of crews.
When it happens, it's bad, of course, but it's a very rare occurrence in airline operations, fortunately. Just for starters, there are two of you in the cockpit, plus one or more cabin crew, so that someone might notice your being over the limit, especially when you have all just been on a night-stop together. In private flying you are often alone in the cockpit, with no one to notice that you might have been drinking.
I remember once when I went over to have a word with the pilot of a Cessna 195 Airmaster, after he had done a rather neat "crop-duster" style approach at our airport, one with a lot of student traffic. When he opened the door I could see all these shiny alloy beer cans rolling around on the floor behind his seat!
I just complimented him on his classic round-engine machine and took my low-time, non-rich self off in a different direction then, because there was not much sense in trying to get his attention, a nobody talking to a somebody, a somebody who was probably half-lit. That was 40 years ago, and down South, when a different approach to all this was taken.
He later dinged in, when the accident report autopsy showed him positive for alcohol, but there he was a rich guy who flew alone and did as he pleased. There was no way he could have got away with that flagrant behavior when flying for an airline, even then.