Ever since Qin Shi Huangdi ("First Huangdi of the Qin") made up the title 皇帝 (huangdi), Vietnamese and Japanese rulers quite consistently claimed to be "emperors" as well. The Japanese made up their own title, 天皇 (tennō), and the Vietnamese proclaimed that their land was ruled by their own huangdi with the poem The Southern Land's Mountains and Rivers. Korean rulers seem to have been conspicuous exceptions, given that they only started to use 황제 with the establishment of the westernized Korean Empire, following Japan's footsteps; before then, they only used the title 왕/王 (wang) which had long been demoted to a princely nobility title elsewhere.
Was there any linguistic or historical reason for this? I feel like this is probably the only place I can ask this question because I doubt there's anybody with adequate knowledge of the Korean language on History Stack Exchange.