Rationale behind lack of transparency regarding thesis defence committee
In some cases, there is an external bureaucratic entity within the university that imposes such rules on departments, along the way insinuating that an "outside examiner" (even if there is none qualified) is necessary to "maintain standards", because (supposedly) otherwise departments might grant crappy PhD's... But, at my university, such secrecy is not required, in fact, nor is prior communication of student with committee prohibited. After all, wouldn't such communication help everything?

Similarly, some faculty suspect or even explicitly accuse other faculty of having low standards, etc.

Oddly, my university gives PhD students exactly one attempt at a "final oral exam" (=thesis defense): if they "fail", they are terminated. That is, there cannot be any "message" to go back and do more work... which seems to me bizarre.

In my opinion, the scenario in which a student presents a thesis as a fait accommpli with a short time-table to do trivial revisions (as opposed to altering the scope, or, heaven forbid, really learning more things to do a better thing...) is a bad thing. That is, interacting with the committee only at the end of a multi-year project strikes me as bizarre and not useful. All the worse if the committee's composition itself is a last-minute surprise, so that nothing positive could have been done.

But, yes, some of my colleagues apparently believe that there are universal absolutes, so that the notion of discussion and negotiation is what is incomprehensible or corrupt. Indeed, as with some coursework, it seems that a large part of the "test" is to figure out what the "test" might be about. I suppose there is a watershed of opinion about whether such a meta-test isn't at a seriously more-expert level than the literal material. Certainly at the PhD level, the meta-questions can easily become so serious that experts don't agree, so it seems unreasonable to me to require novices figure this out on their own.