Even when the margins are insurmountable, you still have to play out the whole game. That’s what gives democratic institutions their legitimacy. Demands to short circuit the process are demands for invalidating the entire system.1
The fact is that there are other offices and even propositions that get voted on during primaries. It’s not just a race for POTUS. People dropping out before the process is complete guarantees diminished voter turnout, which ensures the protection of the status quo in Congress, in state government, and in local government. Either have faith in the democratic process or stop pretending you care about democracy.2
Contested primaries make the eventual nominee all that much stronger, especially when they end up subverting pre-crafted media narratives.
As much as I despise the sports analogies lazy writers make with regards to the primaries, there’s a lot in common in this regard: a nominee that fought hard, ran their best campaign, and won is going to look much stronger than a nominee who is selected by default. It’s like when a team makes it to the finals in a tough conference compared to when a team makes it to the finals because everybody in their conference is a joke.3