They just can’t help themselves. In any other era, you’d expect to see one or two back bench Republicans, and one or two obscure Democrats, make huge, embarrassing, gaffes. It’s just the nature of the beast. When neophytes spend a ton of time in front of cameras, stuff happens.
However, the modern Democratic Party has really managed to obliterate the old way of doing things. It’s almost impossible for them to let a day go by without one of their standard bearers – their best, brightest, and most famous faces – saying something just incredibly dumb.