I was wondering why e4-e5 openings have different names depending on the next move, but 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?
Back in the day where most openings got their names, 1...e5 was much more popular than the Sicilian, so it made sense to distinguish between the King's Gambit, the Italian and so on as those were the openings everyone was playing.
The Sicilian however, was much more obscure until the 20th century, so people felt no need to be more specific about it.
It's a bit similar to the distribution of ECO codes, where openings that were popular at the time they were established get a wider range of entries.
Your premise is wrong, in my opinion.
1.e4 e5 does have its own name that the rest are variations of -- it's the Open Game (although most people just refer to it as "1...e5", but that's still refering to the opening as a whole). And lines within the Sicilian do have their own names, you mention the Najdorf and the Accelerated Dragon, comparable to say the Ruy Lopez and the Italian.
So I don't see a real difference.