Why don't they make the liner just a few inches longer, so they can
touch the bottom and leave some over lap at the top?

I modified ours, with a cord around the top edge lip of the bin, using
a spring to pull the cord tight - as a means to try to grip and retain
the liner and hold them open. As soon as you get any weight in the
bottom it is dragged down and out.

My latest fix, is to put five layers of 1" thick expanded polystyrene
covering the bottom of the bin, with a sheet of thin alloy on top, all
retained in the bin base via threaded rods bolted through to the base.
It needed to be light, to survive the mechanical emptying process and
rot proof. So far, it seems to have solved the liner issue.