Thats... thats not how logic works. If 100% of the time B occurs, it was preceded by A, then it is fair to view A as a possible cause, even if sometimes A occurs without a subsequent B. Concept is known in mathematics as "neccesarry and sufficient". A would be neccesary for B to happen, but not 'sufficient' (meaning there might need to be something else besides A to cause B, but the A is always needed, i.e. neccesary)

Real world example: Exposure to the cold virus is a NECCESARY condition of catching a cold, but not SUFFICIENT conditions. Everyone who gets sick with a cold was 100% definitely exposed to a cold virus (it is a NECCESARRY precondition). However many people get exposed to the cold virus and mever get sick (exposure alone is not a SUFFICIENT precondition to getting sick).

Just like the fact that not everyone who gets exposed to cold get sick, does not disprove the theory that viral exposure causes colds, the fact that not every poor person in existence is a mass shooter does not in any way disprove the theory that poverty is a cause of mass shootings.