This is not in accord with my experience (outside Pennsylvania).
Generally, when real property is held in joint tenancy with right of survivorship, the title is transferred upon death by operation of law outside the probate process. Title to the property is made marketable by recording a death certificate together with an affidavit of someone who states that they have personal knowledge that the person in the death certificate and the person on a particular deed are one and the same person in the real property records in the county where the property is located.
The only possibility that I can think of that makes some sense, and could explain things, is that Pennsylvania is one of the only 17 U.S. states that still has an estate or inheritance tax, and is one of only five of those states without a large exemption for smaller estates (the Pennsylvania inheritance tax is 12% of the value of assets that pass to a sibling). It could be that probate is necessary to fix the inheritance tax obligation and adjudicate that it has been paid in full, so that the real property is free of a state inheritance tax lien.