As far as who chooses the candidates I would think that the Media is more responsible for that than anything/body else.

And the media is controlled by the corporations...who also control the DNC and the RNC. Look around and you will find that practically nothing is controlled by "The People".

The Dems are waiting for the convention to determine who will be their candidate.
No, the candidate has been chosen and it's Joe Biden. Sanders is using this time to help with the Covid-19 response from the bully pulpit provided him by his candidacy. Biden is taking a victory lap and struggling for air time.
Good coffee, good weed, and time on my hands...