Some because they are very close to be real and others invoke the true sci-fi core of "What If?". Last night I watched the movie called "Womb" released in 2010. It is gray, cold and too close to become reality to be considered "Sci-Fi". As a Sci-Fi film, it hits the foundation of Science and Human morals, what is considered acceptable and what is not. Many times, in real life, these divisions are grayed out when it is you that lives that situation. There are no gimmicks, space ships, advanced wizardry gadgets, androids or aliens but just Science and secular humanity. Interesting and thought provoking Science-Fiction. This trailer does not do justice but catch it on cable or rent it if you can and watch it on a sunny day.
P.S. - Back in the late 1970s or early 1980s there was a similar plot Sci-Fi movie but did not remember the title. I think this film was inspired by it.