Musicians get sued a lot for stealing other musicians work, but movie writers also riff on a lot of the same ideas of one another, yet I never hear of screen writers/directors getting sued. Why is that?
One reason could be because of the scènes à faire doctrine. Many of the things you notice as similarities are not infringement.
I don't know the counts of lawsuits for music infringement vs screenwriting infringement. There may just be more songs than screenplays. Screenwriters/producers do get sued, though. See Cinar Corporation v. Robinson 2012 SCC 25.
There are many more possible screen ideas than song ideas. At some point, it is inevitable that songs will sound similar to each other.
For example, there are only 12 musical notes in total and only 7 of those notes (called a scale) sound good together at any one time. Most melodies only use notes from the scale from which it's constructed. Accordingly, there are only 3 distinct (harmonic) chords (combinations of 3 or more notes from a scale).
If you consider how many songs that are being written using the same 12 or 7 tones, it is simply a mathematical / combinatorial fact that many songs will eventually sound similar to each other.
Ideas are not copyrightable, only "expressions" of ideas.
Two screenwriters may have essentially the same idea, but if they are expressed differently, there is no infringement. The example that is often used is "Romeo and Juliet" vs."West Side Story."
There are just many more ways for screenwriters to express their ideas using numerous words than for musicians using twelve notes. A screenwriter using "reasonable care" can probably avoid charges of copying. That may not be true for musicians.