Why is personal experience not proof of something?

I have always been interested in this question, because it would seem that personal experience is something that happened to you, happened, and you realized it, or through some actions you came to something, and that technically it is quite possible to happen to another person like you, why then when discussing with the same person you can not mention your personal experience as confirmation of certain things?PS I will also add: what does "personal experience" mean in general in the full understanding of this meaning? Because, in fact, any discovered, studied, investigated thing that led to this or that conclusion is considered a "personal experience", even if it was done in the walls of some laboratory or somewhere else, and in this case the question still follows-probably from the category of philosophy, how are the boundaries of "personal experience" defined in general? Where is the line that separates "personal experience" from non-personal?