It's easy to reach that conclusion if you have never been active in a party. The democratic party does have a good local ground game that is run by volunteers who actually have beliefs and principles. The problem is that at the end of the day the candidate who wins in the one who can pull in the most money, and that's true no matter what. Where do you see a situation where an independent candidate can pull in enough money to win an election? The message is important, but getting the message out is what brings in votes. Just as the democrats need a more coherent message than just "we're not Trump", an independent needs a better message than just "I'm not a democrat or a republican" - and they need backing in order to get that message out.
There have been some democratic candidates who have been able to get some local traction without taking special interest money, but that's only possible if they had a large war chest of money ready before they announced their candidacy. An independent would face the same challenge.