Because they have no standards whatsoever.
They've nominated a guy who served two years in the U.S. Senate (Obama), an old man with dementia who had been in Washington for over 50 freaking years and was corrupt as hell (Biden) and the most incompetent, jibberish-spouting clown to ever hold the office of Vice President (Harris). And that doesn't even include arrogant blowhards like Gore and Kerry. It's a party that nominated Adlai Sevenson to run against the hugely popular General who helped win WWII, Dwight Eisenhower, and after he got blown out, nominated him again four years later.

Then there was George McGovern, a far leftist (but at least a patriot). Only JFK was a somewhat impressive candidate. Oh and maybe Bill Clinton, if you ignore his pecadillos and simply amazingly bad in choosing a spouse.