Presumably the left and right pitot-static locations are similar, thus equally likely to have a problem.
Is the standby system positioned in a significantly different location, thus less likely to simultaneously suffer the same problem?
Even with a viable third system the crew would have to decide which system(s) were more reliable; normally an airdata comparator would flag a discrepancy between the primary systems requiring the crew to choose the best (matching) ‘two out of three’ systems. However this is a much more serious problem if two systems are unreliable.
Such a situation could defeat most 2/3 comparators; the technology is deemed to have failed, thus safe flight depends on human skill - a situation which technology is unable to resolve, yet where technology is used because the human is similarly unable to choose with certainty. ( AF447, and previous A330 events - airdata, Sweden CRJ - attitude).