Your statement may be correct, but in fact they did - Formally. Every synagogue has within it a throne or chair called 'the Seat of Moses'. When the chief rabbi sat upon the Seat of Moses his words were considered to be infallible.
Of course, Yeshua put an end to that. It wasn't true then any more than it is true now.
Matthew 16:19
I will give you the keys of the kingdom of heaven; whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.”
1 Timothy 3:15
"...which is the church of the living God, the pillar and foundation of the truth."
Matthew 18
15“If your brother or sisterb sins,c go and point out their fault, just between the two of you. If they listen to you, you have won them over.
16But if they will not listen, take one or two others along, so that ‘every matter may be established by the testimony of two or three witnesses.’d
17If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector.
18“Truly I tell you, whatever you bind on earth will bee bound in heaven, and whatever you loose on earth will bef loosed in heaven.
Bishop St. Cyprian the Martyr of Carthage in 252 [Epistle 59:14 in PL ],
After such things as these, moreover, they still dare--a false bishop having been appointed for them by, heretics--to set sail and to bear letters from schismatic and profane persons to the throne of Peter, and to the chief church whence priestly unity takes its source; and not to consider that these were the Romans whose faith was praised in the preaching of the Apostle, to whom faithlessness could have no access.
1. This comment is irrelevant to the story at hand, since there is no reference to chief priests in this passage, but plenty elsewhere in the gospels, and even references to "the chief priest and the pharisees" which suggest that these terms are not interchangeable.
2. Despite that, I'd be interested in any evidence you have that "When the chief rabbi sat upon the Seat of Moses his words were considered to be infallible."