Because I, for one, didn't know it was happening and therefore could not participate. My chance of having my opinion count was zero. So pretty much by definition the poll is not representative of the universe of people-who-include-me. And by induction of all the other people who had no chance to participate.
If you want to know more, I recipient looking up the 1936 Literary Digest poll for US President, one of the most famous screwups caused by non-random sampling: https://www.math.upenn.edu/~deturck/m170/wk4/lecture/case1.h...
its the same with US polls, everyone having an opportunity is merely theoretical. Lots of people are unable to votes for all sorts of reasons.
Perhaps explain how you are defining * opportunity to participate* .
Previously you said "any self selected polling is meaningless"
Now you are saying "Self selected polling is meaningful when people who didn't self select had the opportunity to do so" .
Ok. So by that definition the poll I linked, everyone had the opportunity to participate in the debate, theoretically. What makes it meaningless ?
PS: Fixing democracy ect is not interesting to me in this context, I want to focus your claim self-selected = meaningless.