The short answer is that the word disgust was imported into English from French with the prefix already attached, but the word without a prefix was not also imported.
If you look at the etymology of the word disgust, you'll see that it comes from the 16th-century Middle French word desgouster, which was formed from des- (a negative prefix) and gouster (to have a taste for). By some accident of history, only desgouster was adopted into Middle English and eventually the modern English disgust, though you can see evidence of gouster in the rarely-used word gustatory.
Both of these words continue to exist in modern French, with the letter 's' being turned into diacritics: goûter means "to taste" or "to like the taste of"; dégoûter means "to dislike the taste of".
So why wasn't gouster also imported into English? Again, it's probably just an accident, but there are two possible contributing factors:
- As @rjpond notes, the word gust already existed as a Norse import having a completely unrelated meaning.
- There's another Middle French word, taster, that originally meant "to touch or sample" that was imported into Middle English. Since then, it has evolved differently in each modern language: in modern English, taste exclusively means "to sample by mouth", but in modern French, tâter exclusively means "to sample by touch" or "to feel out". So the modern English "to taste" now means the same thing as the modern French "goûter".