As a museum professional, I don't agree with a couple of points:
If you want science for grownups, you have conferences.
I work at a history museum, and we serve both students and adults: whole range of people. Conferences aren't designed to communicate science (or any specialized topic) to a wide audience.
Also, that it is for kids doesn't make it impossible to enjoy as an adult, especially if it is about things you are unfamiliar with.
This can be true, but children and adults learn differently. We have lessons and interactives that are designed for both, and activities that are geared towards kids. The way we write information for children in our programming is very different from what you'd see with adults, because of how we have to break the information down in ways that is understandable to them.
If you go to a science museum and think "only a kid can enjoy that". Then the problem is not that it is a place for kids, it is that it is just bad. It is a thing Disney understood very well, its classics may look like they are for kids, but they are actually enjoyable by everyone, and it is a big reason for their success.
I don't understand this line of reasoning: if a science museum appears to be designed for kids, there's likely a reason for that: they're working to communicate science to kids. That doesn't make it bad: it might just mean that they've put a lot of focus on their primary audience. Disney isn't designed for kids: it's designed for families, and they put a lot of time and energy and resources into that design. (Museums can take a leaf from their book and strategies!)
As for art museums, the problem is that they are usually just exhibitions, and to be honest, that's boring, especially if you are a kid. That's unlike a science museum where they actually try to teach you science. It is only interesting if you are already well into that kind of art, and most kids aren't (yet?).
History museums are kind of a middle ground as they can do the double duty of teaching history (mostly for kids) and showing off artefacts to people who are already into that (mostly for grownups).
I think both of these points are overly broad, and every institution and every exhibition is different: it all comes down to how well they design their programs and exhibitions. There are plenty of art museums that go beyond a mere exhibition.
As for history museums being a middle ground, I don't agree with that at all: kids are fascinated by physical objects! Adults love to learn about the history behind those objects! These aren't mutually exclusive things. It ultimately comes down to intent and installation and implementation.