If it was an accident, then why does the CDDL even exist? What does it offer that MIT/BSD don't have, other than being incompatible with GPL?

Like Mozilla, per-file copyleft and clearer patent licensing, IIRC.

Some prominent former Sun folks have publicly claimed the GPL incompatibility was intentional, other equally prominent folks have disagreed.

This argument could be made for any license that’s similar to another, I don’t think it’s fair reasoning. Why does 2-Clause BSD exist? Why does Apache license exist?

That's a good point, though honestly I never could understand why you would use anything but Apache (for permissive) or (A)GPL (for copyleft). Other options seem like oversights (BSD/MIT don't cover patents) or cutting off your nose to spite your face (WTFPL). I'm sure there's nuance, but I can't seem to see it.

MIT license and BSD license seem to long predate Apache.

GPL and AGPL differ vastly in a few important aspects that make GPL pretty OK for commercial companies, and AGPL an absolute no-no.