I was wondering why e4-e5 openings have different names depending on the next move, but [Sicilian Defence](https://en.wikipedia.org/wiki/Sicilian_Defence) continuations are all just variations of the Sicilian Defence. Is that because they are frequently transposed into one another, or is there another reason?

For me personally they do differ a lot. The [Accelerated Dragon variation](https://en.wikipedia.org/wiki/Sicilian_Defence,_Accelerated_Dragon) and [the Najdorf](https://en.wikipedia.org/wiki/Sicilian_Defence,_Najdorf_Variation) have different piece coordination and have a lot of theory for a simple "variation."