1. Because he has zero shot on winning. Sorry, with the rules, as they are now, if you don't have a party apparatus to support your campaign, you won't win. Thus, significantly increasing the chance that Trump wins.
2. Obama was in government long before he was a Senator. Plus, community organizing produces a f**kton more useful governing experience than running a business does. Running a business is easy, tell somebody to do something or they are fired. As Trump has found out, and every other businessman that I've seen run a government finds out quickly, it's a lot harder to get people to do something if they don't have to do it. You actually have to get them to work with you without threatening them. Community organizing is doing exactly that. Getting people who don't have to do something to do something. Even then, it only works if you have another side willing to actually govern.
3. Beto O'Rourke is having funding issues. He has at least won a state office and has some governing experience.
Honestly, I don't understand why you have such hard ons for these guys that have not shown they can organize anything without the power to fire anyone. Trump tries to do that now and look at the success he has had. I will say though, Schultz isn't a tenth of the moron that Trump is.