The Democratic primary electorate is probably the segment of the population with the strongest negative impact around the 2016 election hacking that was evoked in the grandparent post, so while true your comment reinforces rather than negates the point.
Parties don't wheel out candidates, candidates seek the party nomination. Candidates who have done so in the past and whose on-paper qualifications and public esteem have increased since then often remain inclined to do so and have a basis for expecting they might do better.
Many party voters are also inclined to vote for these candidates; some of these are long time supporters.
I'm more surprised that candidates whose resume includes only some local politics, a few terms in the House, and a failed Senate bid throw their hat in to the Presidential ring than that those that spent 36 years in the Senate and run for President twice before, serving for eight years as Vice President after his most recent Presidential run, do so.