Except people voted for Humphrey in the primaries. LBJ decided to not seek a second term after Gene McCarthy got over 40% in the first primary-and the press turned on LBJ.
Many states have laws to prevent Biden from doing this and Kamala may be forced off their ballots.
Humphrey didn’t contest any primaries. A lot of states didn’t have primaries, while in others the governor ran as a “favorite son” in order to turn the delegates over to Johnson and then Humphrey. The young progressives in the party were so appalled that they changed the system and since then no candidate has been nominated without contesting the primaries.