Because you can’t trust the hearer. Besides Jesus and the apostles didn’t record everything they said or did in the Bible.
All messages require a communicator and an interpreter.
And besides we have to guard against the interpretations of the age and against cultural biases, which is why all Apostolic Christians look to Holy Tradition to find what the Scripture mean.
Languages change, cultures change, so people are liable to misinterpret the scriptures for their own benefit.
Not to mention, the apostles and their immediate successors ended up waging a pitched battle with Gnosticism from the start, which led to the codification of Tradition as a protector of Scripture.
If it weren’t for Tradition, how would we know that the correct gospels aren’t Thomas, Peter, Mary, and the Egyptians?
Look in our own time how liberals have twisted the scriptures to promote homosexuality, socialism, etc. That’s why holding fast to Tradition is important.
Sola Scriptura offers no defense against the re-emergence of Gnosticism in our own time and those who are trying to trot out the heretical gospels to confuse the faithful.
Tradition defends and confirms the Scriptures.
I must also make the distinction between traditions with a small t (i.e. customs that can be done away with or enhanced) and dogmatic Traditions with a capital T. The Trinity, they Hypostatic Union being two key ones that Protestants believe in.
John 21:25 Now there are also many other things that Jesus did. Were every one of them to be written, I suppose that the world itself could not contain the books that would be written.
The following link has dozens of scripture verses showing the role of tradition in Biblical times. http://www.scripturecatholic.com/oral_tradition.html
Because you cant trust the hearer.
I trust the teller.(The Living Word)
Is the papacy the One and Only? You tell me not to trust myself.
I am the way and the truth and the life., but only through the Catholic Church? Surely ye jest!