https://en.wikipedia.org/wiki/Self-handicapping
E.g. if someone in college does poorly because they got high all the time, maybe the reason they got high all the time was that they felt (rightly or not) that they would have done poorly anyway, and now they have the drug to blame. And maybe those who smoke a bit and still do well, do it sort of to have the excuse in reserve.
I've not much experience with cannabis, it's far less popular where I live than the US, but I can certainly see that alcohol is used as this sort of "flexible excuse", both short term and long term.