The fact that there aren't any serious contenders - and no, RFK is not a serious contender - during years when a president is running as an incumbent isn't new or different. Reagan ran without meaningful opposition in his primary 1984, Clinton in 1996, Bush in 2004, Obama in 2012 (as you pointed out), Trump in 2020, and now Joe in 2024.
In either case, how parties run their primaries and select their candidates -is- up to those parties - that's how the founders built it to work.
Perhaps you'd be less angry if you were less ignorant about the history of our nation?