I think a lot of coaches have a default "don't trust" for parents until they learn which ones they can trust. I'm not saying I agree with that, but having coached another sport, I understand it completely.
Trust has to go both ways, and I've been reading this forum for years, and talking to parents for years, and I'd say that most parents don't allow coaches any amount of trust by default. They won't trust a coach until they've shown that they're worthy of it. I think that's human nature, but it's also harmful to the relationship, and it feeds this feeling that the coaches should communicate more than they do.
In short, until each side gives the other side some benefit of the doubt, it's going to stay like this.
I can't speak for all parents but I also run a child's competitive activity and I'm a parent with kids in many different activities. I think that most parents don't initially distrust coaches, but they are neutral about them, until the coach has either shown to be trustworthy or not trustworthy. We all know that some coaches are abusive and if most parents assumed coaches were abusive, most parents would be watching every minute of practice.
But I agree with you that many coaches seem to immediately distrust parents. And as a parent, I find it's hard to trust someone who obviously distrusts you and thinks so little of you. It's also impossible for the parents to earn trust from the coaches if not given the opportunity to do so.
Dd's coaches don't communicate with me at all. I read here about yearly conferences, written progress reports, team meetings - I have not gotten any of that. The gym obviously communicates about practice schedules, meet schedules, and money due, but I have not exchanged more than 10 words with dd's coach this year, which were probably hi, thanks, bye, see you later. Same with last year's coach. I don't have the coach's email address or phone number to text. I guess if I wanted to set up a meeting with the coach, I'd have to try to do it through the front desk of the gym - but I'm not sure how that would work since the time I emailed the front desk to ask a quick administrative (not specific to my dd) question, I got no response at all.
I am getting the message loud and clear that the coaches want me to just pay money and bring my kid and otherwise butt out. And I do it, but yes, this has not built up my trust in the coaches.
I know from the coach profiles on the gym's website, dd's coaches have all been competitive gymnasts at high levels. But some of them have been very young, and not had much coaching experience. Being an expert in gymnastics does not necessarily mean you have an understanding of child development and understand what else is going on for the kid, nor that you're at teaching the skills you once were able to do.