People that participate in our democracy not knowing the basics of how it works is getting exhausting. The DNC doesn’t “pick” canidates: voters do. The DNC absolutely throws its support behind preferred candidates, but voters still make the final decision.

“Pick someone else” also smacks of “I want someone else to do the hard work and find a viable alternative candidate for me, because I can’t be bothered to put actual effort in to the political process.” Nobody is stopping you from campaigning for your preferred presidential candidate in the primary – or even running yourself if you’re eligible.

A lot of people do, but they won’t. That’s not really what the original commenter was asking though.