> [...] despite them being constructable from the basic Hangul codepoints.
Unicode strives for the round-trip compatibility with source character sets, and in this case KS X 1001 (KS C 5601 at that time) is a main culprit: it had 2,350 (out of 11,172) common syllables precomposed. But it happens that Korea had supplementary character sets beyond KS X 1001, which were subsequently added to Unicode 1.1 (up to some 6,000 characters), before it was decided that having an algorithmically derived section of all 11,172 syllables is better. This whole situation is now known as the "Hangul mess".
Unicode strives for the round-trip compatibility with source character sets, and in this case KS X 1001 (KS C 5601 at that time) is a main culprit: it had 2,350 (out of 11,172) common syllables precomposed. But it happens that Korea had supplementary character sets beyond KS X 1001, which were subsequently added to Unicode 1.1 (up to some 6,000 characters), before it was decided that having an algorithmically derived section of all 11,172 syllables is better. This whole situation is now known as the "Hangul mess".