Because the 'church' says so.
Unprovable to what standard? The Church, "the pillar and foundation of truth"? Or Elsie's infallible standard?
Because the 'church' says so.
Yes, the Body of Christ (the church of the Living God) is supposed to be where one finds the truth supported and upheld, but nowhere does Scripture say the church is the creator or inventor of the truth. It's not up to the church to decide what is or is not the truth - that is what God gave us His sacred, inspired word to do. If the church presumes to make up what is the truth to be followed and believed by all Christians, then they are exceeding their role as the buttress and support of the truth. If any church stops being that, they cannot be the church as Scripture defines it. When Jesus spoke about taking a brother one has an issue with to the "church", I think it is pretty obvious that this meant their local congregation of which both were members - else how could their decision hold any weight? I highly doubt Jesus had a monolithic, central organization in mind when he gave this remedy for personal disputes.
We have many examples of the failure of the Roman Catholic Church to uphold the truth as God revealed it. Rather than hearing and accepting this, many Catholics prefer to stop up their ears, close their eyes and repeat the mantra that ONLY their church is the true one and whatever they say MUST be true. "Just trust us on this - wink, wink."