When first encountering these ecosystems and looking at the various pieces they contain, one may repeatedly ask: "why didn't they just use <off-the-shelf solution> for this problem instead of writing this component/subsystem from scratch"?

The answer is often that the battle-hardened mature off-the-shelf solution did not exist at the time the code was written. You're doing software archaeology.

That's only half the answer. These large investment banks' value-add is partly that they can integrate everything they know into these closed-world environments (kind of like a Smalltalk image), which is something that simply isn't done in the wider world because you can't accrete it out of smaller pieces and it doesn't make sense at all for smaller entities.