A soundtrack, also written sound track, can be recorded music accompanying and synchronized to the images of a motion picture, book, television program or video game; a commercially released soundtrack album of music as featured in the soundtrack of a film or TV show; or the physical area of a film that contains the synchronized recorded sound. Wikipedia, Soundtrack
Silent films almost always featured music:
Showings of silent films almost always featured live music, starting with the guitarist, at the first public projection of movies by the Lumière Brothers on December 28, 1895 in Paris. This was furthered in 1896 by the first motion picture exhibition in the United States at Koster and Bial's Music Hall in New York City. Wikipedia, Silent films
Soundtrack industry is considered to have been started around 1930s. So, I assume that the music featured in silent films isn't called a soundtrack. Am I right? Why is this (or the vice versa) so (note that the emboldened wikipedia sentence gives a definition that would actually call them soundtracks)?