I think that's the key point. It doesn't matter what the benefits or losses are.. it should matter whether I actually get to enjoy a democratic say in my government. Which, it distinctly feels like I won't if the EU carries on as it is. And note, when I say "my government" I don't mean Britain, so not on that pure sovereignty/independence. If a group gets to pass laws that say what I can or cannot do, establish taxes that I have to pay, and in general acts to control the society around me.. I really, really want a way to vote on that group that's actually meaningful.
Not a "vote for some vague personage who just seems to be there for PR purposes, while the work and legislation is handled by other people entirely and don't rely on whether I want them there or not".
Getting a pat on the head with "You'll be better off economically, and you'll be able to travel within Europe better. Don't worry, we know what you want, we're the smart people here, you can trust us to have your best interests at heart. Honest. We just won't tell you who we are or how we reach our decisions." is not something I'm all that comfortable with.