You are asking a reddit style question.

The entire world works around the legitimacy of authority.

Yes - the quality varies, and pollsters don't have a high degree of institutional authority.

But some do.

If you want to learn about 'institutional legitimacy' - go visilt Pew.

Pew is an extremely well regarded institution. They are extremely good at polling. Go look at their methods.

They are a long running institution with serious professionals, big budgets, people who really know waht they are talkinga bout.

It's the difference between getting diagnosis from 'a neighbour' vs a 'doctor' but in this case there is no specific qualification.

Some 'respected pollsters' are basically insiders who have been doing it for a long time, with relationships and credibility - others, more institutional.

Check out Pew, they are probably he closest thing to institutional professional as possible.

Political polling is inherently hard - they will be the first to admit.