I absolutely understand that the ranking at the end of regular season is what matters before the tournament participants are selected. But Why? How do we know pollsters will get it right?

They never get it right on pre-season polls.

Was Florida really Expected to be the 4th best team? Why? Was LSU really expected to be the 7th best team? Why?

Last year, Ole Miss was ranked 3rd pre-season. They finished dead-last in the SEC.
In 2022, State was ranked 4th pre-season. They finished dead-last in the SEC.

Why does a pollster believe a team that lost 2 to Austin Peay, 1 to Central Arkansas and 1 to South Alabama is the 15th ranked team? Same pollster that had LSU and Florida incorrectly ranked in the Top 7.

Far too often where you start is nowhere near where you finish. My point. Ignore any poll prior to May 1. By then you will have an idea who the best teams are.