If the post cannot go down further, then it is likely hitting the water bottle boss, but in any case
It's not designed to be supported from the bottom and that extra length is not needed.
There is a minimum insertion line on the post, which shows how much has to go into the frame, usually about 4".
As long as that much or more goes into the frame, inches can usually be cut off
and still leave enough to raise the saddle much higher than you ever would want.
I often cut a couple of inches off the post just to save weight.