Something that really frustrates me is people who are more expert than me but can't justify their pronouncements. I had a high school friend who I'm sure was smarter than me claim, "No one does object oriented programming the right way [except him]," but when pressed would just deliberately play games and mystify. Most DBAs I've worked with had their preferred way of doing things, but were incapable of answering Why? It's not easy working with these people. I've come to really value a colleague who can
teach---that is, who can explain why they choose what they do.
I'd say anecdotally 2 out of 5 times I've encountered this it's been someone covering up for their own lack of understanding. These are the worst and the most inflexible. All but one DBA I've worked with was this way. It's the natural result of cargo-culting.
2 out of 5 times it's because the person doesn't have patience to explain all the background you'd need to understand, or they don't trust that you have sufficient background. Sysadmins do this all the time. If you're smart and already pretty familiar with their field, it's very frustrating. This isn't as bad as the former case, but it indicates personnel conflict and lack of trust. It's understandable though, e.g. answering technical questions for business people. I've gotten pretty skilled at giving non-jargony, intelligible, short, "popularizing" answers, that also invite further questions if desired. But it's not easy.
And 1 out of 5 it's because people with experience have forgotten the reasons. I've been there too. Just the other week on a Rails project someone wanted to use an ActiveRecord default_scope of "where deleted_at is null", and I had felt the pain of that on past projects, but it took a while to dredge up the details from my memory.
Most of the above reasons don't correlate with age/experience, but the last one does. Experienced people have so internalized certain lessons that they've forgotten the reasons behind them. You could say this is a bit like Michael Polanyi's "tacit knowledge". In this case, hopefully they still have mental flexibility to recognize that nothing is 100%, and if you give them time to remember and communicate their reasons, you can have a reasoned discussion and make a good decision. But recognize that making people think in this way often "feels like work." You are asking them to exert themselves, so it's important to communicate in a way that isn't challenging their knowledge but asking them to share it more deeply.