Primary elections are private matters, “nominee of the Democratic Party” is not a political office.
Right, so you don't get to pick your leader.
You pick your leader in the presidential election.
Being a nominee of a major party is not a requirement to get on the presidential ballot.
The problem is that primaries are giving people the illusion of a choice. This country hasn't been a democracy or a republic for many decades yet it's constantly held up as something we are protecting. People decide who the president is as much as an employee of Google or Meta decide who their CEO is.
This is democracy in name only. There are much better alternatives in existence. Don't defend the current system, build a new one that is more resilient and fairer.
People's opinions changed after the primary. This doesn't have anything to do with voting booths one way or the other.
Do British citizens get to vote on who each parliament party puts forward as their leader?