Why were so few interwar seaplane carriers built?

Most seaplane carriers could be converted from available hulls, with HMS Ark Royal standing out as one of the few purpose-built examples. Their lower cost may have made carrier advocates wary that governments would mistake them for cheap replacements despite the very different missions. Used correctly, however, they could complement fleet carriers as forward aviation support ships in ways comparable to the later support role of HMS Unicorn. France, the United States, and Japan all had imperial or oceanic regions where such mobile infrastructure could have supported dispersed operations. Their interwar rarity remains an open question rather than a problem with a single stated answer.