This is a good question. The Lord himself has recommended a Guru to reveal the Truth. “Gnanis and Tatvadarsees will reveal the secrets to you” said Krishna to Arjuna..
We have eyes and face but still we need a mirror as a medium to look at our own face. Similarly we need a Guru. when we are thirsty we cannot drink water from the sea. We need the water which the clouds pour down on us as the rain. The clouds transform the saltish water of the sea into sweet water for us to drink. Similarly, the Guru acts as the medium to transform us to be worthy recipients of God’s bounty. When we loose a vessel in the well. we cannot jump and retrieve it. We need a tool to retrieve it. Similarly, we need a Guru. We can give any number of examples like this.
The Lord and the Guru can be found in two forms. One is Vapu and the other is Vaani. “Vapu” is “The body” & Vaani is “The voice”. Vapu is limited by time and place. Vaani has no such limitation. It is present all the time in the past, present and future. We cannot find Krishna as Vapu now, but we can find His Vaani through Bhagavat Gita even now. It is impossible to find Acharya’s Vapu all the time but their Vaani is external & divine. God has blessed us with ears that cannot be shut unless we choose to do it. So let the Vaani of our Acharyas and the Lord Krishna ring in our ears.