Some years ago at a social gathering, one of us (not me) challenged the rest of us (including me) to name the four national capitals with one-syllable names. The answer is: Bern(e), Prague, Seoul and Rome.
But there’s something deeper here – in the local languages, three of these have two syllables: Praha (cz), 서울 (kr) and Roma (it). Bern(e) is problematic: in two of the official languages (standard (Swiss-)German and French) and in the local language (Bernese German) it has one syllable (but three slightly different pronunciations); in the other two (Italian and Romansh) it has two (and two slightly different pronunciations).
Why do so many national capitals have multi-syllable names? Well, why do so many towns and cities have multi-syllable names? The number of one-syllable town or city names is very, very small.
PS added 1 March. Despite ‘서울’ officially being two syllables, I have only ever heard it pronounced as one. I couldn’t find an equivalent Korean city with a two-syllable name where the first finishes with a vowel and the second starts with one.
I have a vague memory of the Australian swimming coach Lawrie Lawrence getting all excited after the Australian swimmer Jon Sieben won a gold medal at the 1984 Olympics, and saying how much he was looking forward to ‘See-all’. The alternative transliteration of Sŏul is less likely to be interpreted as two syllables.