Also, if the guy or gal has alcohol problems, it would likely be visible on their flying performance over time, it should be noticed during the periodic medicals, etc.
So while a drunk pilot could be the immediate cause of a crash, it is not the only one. If any of those other things I have mentioned functioned as designed (or were in place to start with - not all flying is airline flying!), the accident wouldn't have happened.
If you focus only on the "drunk pilot, case closed", you will never identify deficiencies you may have elsewhere and which have contributed to the problem.