Rationale behind lack of transparency regarding thesis defence committee At my department, a thesis defence committee consists of 3-4 professors (including the advisor(s)), one of whom does not belong to the department (so could be pretty much anyone related to the field).
It is common for a grad student to know who his thesis defence committee members are going to be quite ahead of the defence date (usually the student will know before copies of his thesis are sent to the members for review).

In rare cases, the student can even assemble his committee together with his advisor -- this makes sense for example in the case where the student wishes to pursue a certain research direction after graduating, and he would like a potential advisor or reference who is an expert in the subfield to take a part in his evaluation.

However, the general policy of the university (and of some faculty members) is that a student is not supposed to have the information on who is included in the committee until the defence.

While I understand some potential reasons for it, such as avoiding bribery in any form, those precautions seem quite far fetched at a small department where everybody would probably know if any foul play was going on. Also, if it really were an issue, there should have been anonymity in other examination-related situation during one's studies. From a different perspective, I personally would rather not discover that Euler is on my thesis defence committee when I walk into the examination room (and not because of the startling discovery of the existence of zombies).

What reasons, from your experience, lead to such no-transparency policies?

(I am not facing such a situation, the question arises from a case involving a colleague.)
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.