My ISTP would never dream of helping fix people problems. It isnt that he doesnt care, but I dont think he knows what to do and so would just rather withdraw and avoid it.
He does fix all types of things for me, even though we are now divorced. He put in new light fixtures, fixed stuff in my car, and changed out some electrical stuff in the walls. I think this is how he shows he cares.
I do suspect that ISTPs may often end up in relationships with FPs, who can look sort of FJish at times as we can be pretty sweet. However when unhappy or depressed, we display a LOT of emo, which I suspect most ISTPs would have a very difficult time navigating. Thus it would not surpise me to find out that an ISTP ended up with an FP, who, from the ISTP perspective, had emo issues. I have seen this same problem with INTPs too, in relationships with FPs. So maybe it isnt that ISTP seek emotionally distrubed people-but it seems natural at first, then they get surprised once in the relationship as each side becomes more open and sharing. Sad Fi is BAD. I dunno, maybe that is part of what you see..