In John Williams' case it's particularly funny, since he got his start as an orchestrator! The notable exceptions to this rule on broadway are Kurt Weil and Leonard Bernstein.

As far as I recall Bernstein composed Candide on the piano and let someone else orchestrate it.

bernstein "touched up" the orchestrations, and of course orchestrated the overture, but you're right. Hershey Kay and Maurice Peress did the orchestrations. West Side Story was also not orchestrated by bernstein (except for the orchestral suite)

Yeah, he mentions his orchestrator in this documentary on Empire Strikes Back: https://www.youtube.com/watch?v=NT4_ERNVx3Q

Given the time constraints on getting the film scored it makes a ton of sense to have help in that department.

Orchestration also means something different these days for most film, TV and game composers compared to the traditional definition. Traditionally, orchestration would be taking a piano or short score and expanding it to be played by an orchestra. Nowadays, almost all media composers create full digital mock-ups of the music first with the entire orchestra and then some. Orchestration then is mostly a process of transcription, typesetting and adjustments for live ensemble so the recording and performance accurately conveys the intent. John Williams and Howard Shore are two of the old guard who write short scores with notes for the orchestrator about what to do. Nearly everyone else is writing for the full ensemble and orchestrating as they go. Then the orchestrators translate that to traditional music notation so it can be played.

Yeah

The actual act of orchestration (translating to the different instruments) is an exercise in "backwards compatibility" and musical knowledge. (Because every instrument has a range, a clef, and they play in a range but read on another range, etc, etc because of some weird thing that happened in the 17th century)

So who is actually writing the sheet music nowadays?

generally, as in many things in music, orchestrators work freelance. Music is a low paying field, so the pay isn't great, but its not terrible

If you look carefully in the endless credits scroll you can see the credits for the orchestrator(s), if the composer doesn't do the orchestration themselves. Herbert Spencer, for instance, is credited for orchestration of Star Wars Ep. 4, A New Hope. He worked as orchestrator on a lot of other John Williams movies and was a film composer in his own right as well.