>However, once inside I find that each product is kept together by a skeleton crew or sometimes by people who can only work on it part time since they're handling multiple projects.
This is common, adding people to a project typically makes things less efficient, which is counterintuitive. It's mainly due to communications overhead and developers, "stepping on each other's toes." It also requires exceptional project management of clearly defining tasks, timing of tasks, proper resource allocation, etc.
>It feels like the bus factor for most projects is one (or very close to one).
The bus factor always exists. If the most important person leaves and there are other redundant people on the project, they will still take time to get up to par, and there are a lot of factors, some that you can't mitigate (like raw ability), that go into full redundancy.
>I don't know how I feel about the situation, and management response to my question/concerns "it is what it is".
It's hard to tell from just a blurb. I would stick it out and see how it goes. It might be that they have a bunch of top notch people who can handle it and like their work. If the turnover is really low, this is a good sign. The best teams are always a small group of really quality people. If that is the case, you get to be a part of something pretty rare. It'll take time to figure that out though.