Why I no longer bother to complain to ISPs about spam
The simple answer: reporting spam to ISPs is not an effective way to make the spam stop.
My job is to protect my users (or just myself) from spam, not to help ISPs police their customers and clean up their spam problems. Complaining to ISPs is no longer an effective way to do my actual job, if it ever was; blocking spammers, spam sources, and ISPs is, though. Once I have dealt with the problem, complaining to the ISP is at best donating my time and effort to the ISP; at worst, it makes me a target.
As a result, my reaction to being sent spam is almost always to block the sender. If an ISP's networks are a source a large enough spam to get me to notice it, I generally escalate to blocking the entire ISP, and I certainly don't bother complaining; if the ISP is rotten, the best result I can hope for is to have wasted my time.
The only time when I bother to complain to an ISP is when I cannot block it because it sources too much legitimate traffic that my users want, and the ISP has a reputation of actually doing something effective with spam reports. There are very few such places left any more.
I expect that whether or not users can articulate this, this is why spam reporting rates have dropped precipitously over the last few years. Users aren't stupid and do notice when what they're doing is pointless and has no (good) effect on how much spam they get.