Been thinking about this also. As suggested above, they simply can’t help themselves. But even if they may think strategically as you describe, I think they want the fight. Why wait for the grand confrontation? Indeed, they might just want Trump.
I think we have at least two factors at play here: What ISIS leadership wants and what distant followers of Isis do.
I don't think the leaders have direct control over their followers as would a well organized military organization. Thus we will continue to have occurrences like San Bernadino, but well organized mass attacks like Paris may be something else.