Why is it important to avoid the overuse of the word I in an autobiographical piece?
There are a number of reasons.
Firstly it would be extremely repetitive and very poor English, in general terms.
Secondly it would detract from ant broader context in which the autobiography, for example circumstances and people.
"I" would be the central character in any biographical work, but the author would not wish to overly repeat this for the reasons I have given. AN autobiography is as much about the experiences of the author and how that life has been shaped. Repeated use of the word I would detract from this. One would want to develop other characters in the narrative and how they and circumstances have impacted on and shaped the life of the central character.