Zardnaar
Legend
I think that power fantasy and escapism is a core element of its appeal. Most people feel that they are powerless in real life and few entertainments give them a feeling of powerlessness. With D&D you can have personal agency and shake the foundations of the earth.
I think it's more that it hits a lot of cultural/religious tropes and myths and legends that have been popular for millenia.
A new Sci fi game for example is really only going to appear to Sci fi nerds who also like rpgs.