Gone are the days of "Iris" or "Kiss From A Rose." Those songs were HUGE. And it didn't matter whether you liked and listened to that type of music, they were inescapable. Celine Dion's "My Heart Will Go On" was another smash hit tied directly to a blockbuster movie: Titanic. These songs were played everywhere all of the time. It's hard to put into context for those who have no pre-internet memories, but even if you were a fan of heavy metal, you could not get away from those songs.
Seal's "Kiss From A Rose" was a track that was created specifically for the movie Batman Forever, Goo Goo Dolls' "Iris" was also written and released for a movie, City of Angels, then later released on the album Dizzy Up The Girl.
Today, global blockbusters like Black Panther, do not come with a smash hit like "My Heart Will Go On." The soundtrack was produced by a blockbuster artist, Kendrick Lamar, but again, no song was no where near as ubiquitous as the Dion song.
Pharrell's "Happy" and the song "Let It Go" are both smash hit songs, but are both from Disney movies intended for children and backed by Disney money.
Back in the 90's, a song and a movie were two distinct products packaged together. Not only did the labels push their products through avenues like MTV and Muzak, but the studios invested money in marketing a product to help move units (CDs) off the shelves. The big difference today is that no one is going to buy a CD single or a physical copy of a soundtrack. So why make the investment in a smash hit if no one is going to buy anything?