I have answered your question. Perhaps the issue is not the question but you avoiding the direct responses provided.
Why are we commanded to study Scripture, ...
No one commands you to study scripture. In Matt. 28:20, we are told to "observe ALL I have commanded," but, as we see in John 20:30; 21:25, not ALL Jesus taught is in Scripture. So there must be things outside of Scripture that we must observe. In Mark 16:15 - Jesus commands the apostles to "preach," not write, and only three apostles wrote. The others who did not write were not less faithful to Jesus, because Jesus gave them no directive to write. There is no evidence in the Bible or elsewhere that Jesus intended the Bible to be sole authority of the Christian faith. In 2 Thess 3:6, Paul instructs us to obey apostolic tradition. There is no instruction in the Scriptures about obeying the Bible alone.
On the contrary, we are told numerous times to both study and know the Scriptures. We are told to write them on our hearts and minds (Ex. 13:9; Deut. 11:18; Prov. 3:3; Prov. 7:3) so that we can avoid sinning against God. We are told to study them so that we are able to "give an answer to everyone that asks us of the hope that is in us" (II Tim. 2:15; Psalm 119:15). God's word is "perfect" (James 1:25) and we must know it if we are to obey and walk in it.
Again, you have stated the commonly offered excuse to viewing Scripture as able to make one "complete" and "thoroughly furnished unto all good works" by saying Jesus "taught" things not given in Scripture. NOWHERE in Scripture is that said. However, Scripture DOES say Jesus "did" many other things not written down and that if they could be, the world could not contain all that would be written. That is a far cry from pretending Jesus taught things secretly to certain people and they just happened to be omitted from Scripture BUT everyone must STILL obey it because the "Church" says so. That was the gist of Gnosticism, by the way. Something the early church, including the Apostles, battled against. I do not accept that they left out critical truths from the writings they left for believers.
Jesus DID say to observe all that he commanded and all that he commanded was written down in Scripture so that future believers would also know what Jesus commanded. Jesus told his disciples that he would send the Holy Spirit to them so that they would be reminded of all that he had taught them. What makes anyone think these people would have forgotten to communicate those truths to others? What makes anyone think that, while they were writing their epistles to the churches springing up all over the known world, they would have skipped over any important parts?
You repeat the assertion that Jesus never told anyone to write, yet how else were they supposed to ensure all future generations of Christians knew the Gospel truth? Jesus didn't HAVE to tell them to write things down, he had the example of the sacred writings of the Old Testament already - and he used those writings numerous times in his teachings. How many times did Jesus, himself, say, "it is written"? How many times did Almighty God direct his prophets to write down what he told them? (for example, see Habakkuk 2:2; Deut. 27:8; Isaiah 8:1; Jer. 30:2; Jer. 36:2; Rev. 1:19) Paul clearly gave a precedent, when he said:
For everything that was written in the past was written to teach us, so that through the endurance taught in the Scriptures and the encouragement they provide we might have hope. (Romans 15:4)
Do you suppose Paul forgot to mention all the oral traditions when he said this? Now, nobody is disrespecting ALL the traditions that have developed over the centuries since Christ, just that what makes up the "rule of faith" for Christians can be found in Scripture and, if it is not found anywhere in Scripture, then it is not binding upon a Christian to obey and follow and it is certainly not relevant to our salvation.
One of Jesus' chief complaints about the religious leaders of his day was how they nullified God's word with their traditions (Mark 7:13). He specifically reprimanded them because, "You abandon the commandment of God and hold to human tradition." (Mark 7:8) Now, how would anyone know what was the "commandment of God" unless they had the written word of God? How can anyone, even today, know the difference between what are God's commands and what are human traditions? Without the written word of God, the Bible, we wouldn't have that ability. That is why Jesus stressed the importance of Scripture and it is why the writers of the books of the New Testament did as well.
“I have answered your question. Perhaps the issue is not the question but you avoiding the direct responses provided.”
I’m not avoiding them, just ignoring them, since they weren’t germane to the question that I asked.
“No one commands you to study scripture.”
Alright, this is a relevant answer at least. I have to say that I disagree, and I think anyone who has been studying Scripture well would state otherwise. I see boatbums has already posted some verses to that effect, as well as answering your other points, so I’ll see what your response to him is. I really don’t think this is a hill you are going to want to stand and defend.