For both me (physics) and my wife (history), in the American system, both at strong universities, most of our committee members read most of of our dissertations. For her, in a field where thesis by publication is not standard (your thesis is typically revised into your first book), her committee at the defense focused on questions and comments based on the committee's reading of the thesis more than on the actual defense presentation, which is apparently also normal in the field. In part, I expect that's because the thesis is expected to be built into something important post-PhD, and comments are seen as helpful in that process.

For me, it wasn't quite so apparent at the defense, and I don't know that all members read the final thesis carefully, but most of them had already seen me publish or present most of the research previously, often multiple times. I also know that some (and not just my advisor) did read the final thesis very closely. My thesis was only partially thesis by publication, however, which may have influenced this; it does now have a fair number of citations in its own right, which is somewhat unusual for the theses in the field, and potentially seen as awkward (it means there's significant work in the thesis that I never published elsewhere).

As a caveat, the American system (before current crises) does feel like it can have a two-tier system of PhD students who are expected to remain in academia (we both were) and ones who are not, even at strong universities. Expectations, and attention given, can vary considerably. The American system also tends to have larger and more closely involved committees than, for example, the UK/Irish system.

However, for the form of plagiarism discussed here: if someone had sentences from papers I published years ago interspersed in their work, and they weren't particularly notable sentences, I'm not confident I would notice. Depending on citations and what the sentences were, I'm not even sure I'd mind much, for example, if they were essentially copying a model definition.