All but two states had candidates for president on their Democratic primary ballots (fuck those two that didn't). My state had like 7 candidates to pick from. And then again for the convention, after Biden dropped out, they opened up the field for candidates to declare again and the pledged delegates all voted on the nominee. What you really should be mad at is that there weren't any serious challengers running, but that's the fault of those individuals for not doing so (even it was under private pressure from the DNC it was their own choice to capitulate). You're also forgetting that a measly 4% of primary voters showed up to vote for "Uncommitted," which turned out to be hugely successful in drawing media attention to the discontent of active voters and contributed to Biden's eventual stepping down.