Why can't Quicktime play a movie file encoded by FFmpeg?

Based on this StackOverflow answer I would add -pix_fmt yuv420p to your command like this; one of the comments mentions adding -vcodec libx264 as well so it’s included here:

ffmpeg -pattern_type glob -i '*.JPG' -vcodec libx264 -s 640x480 \
-pix_fmt yuv420p movie.mp4

Or you could use the format filter. This example will use the scale filter instead of -s, and the format filter instead of -pix_fmt:

ffmpeg -pattern_type glob -i '*.JPG' -vcodec libx264 \
-vf scale=640:-2,format=yuv420p movie.mp4

Also elaborated on in the official FFmpeg Wiki under the heading “Encoding for dumb players”; emphasis mine:

You may need to use -pix_fmt yuv420p for your output to work in QuickTime and most other players. These players only supports the YUV planar color space with 4:2:0 chroma subsampling for H.264 video. Otherwise, depending on your source, ffmpeg may output to a pixel format that may be incompatible with these players.