There is no such thing as "apostolic" authority.Why then did those who were called out by God directly as Apostles find the authority to call out replacements? Why did the early Fathers of Christianity agree that such authority existed and was supported by the teachings of Our Lord even before the Canon of the Bible was collected or even written down yet?
Oh, I didn't say they didn't have the authority to call out replacements, so that question is rather non sequitur. The problem with the use of the term "apostolic authority" today is that it presupposes some line of succession such as Rome claims for itself as being the "only" line of authority. This is manifestly not the case, however, since even in the early church, the primacy of Rome - upon which Rome bases its claims as "proof" of its authority - was not established until very late.
If we wish to speak of "apostolic authority" in a way that actually has any meaning Scripturally, then we should see that any local church pastor who holds to God's Word has that same authority. In that sense, the authority is not "apostolic" but simply is true of any pastor.
Why did the early Fathers of Christianity agree that such authority existed and was supported by the teachings of Our Lord
With all due respect to them, who cares? The "early Fathers" were not arbiters of doctrine, nor do they possess any spiritual authority over anyone, other than the people at the time who may have been members of their local church, if they (like Irenaeus, for instance) were a pastor. Pointing to the authority of the patristics is like pointing to the authority of historians - they may have some good or elucidatory things to say, but that's as far as it goes. The patristics indeed were often contradictory, and in many cases, simply outright wrong in what they said, such as Irenaeus and his wacky claim that Jesus was 53 when He died. Often, the patristics were heretics of a rather severe sort, such as Origen and Clement of Alexandria. The patristics had their opinions, but that's ALL they had - opinions.
even before the Canon of the Bible was collected or even written down yet?
That's a rather simplistic way of looking at it, especially in light of Jesus' own promise to His disciples that the comforter would "lead them into all truth" - which logically would include what was scripture and what was not. Frankly, the whole notion that Christianity was ignorant of what was really Bible and what wasn't until 397 AD is not only silly, but is also objectively untrue, in light of the numerous lists of canons that we know of that predate the Council, as well as the simple and common usage of Christian writers across those first three centuries.