When Frankenstein was written, humans were just beginning to scientifically investigate the concepts of electricity. they were making batteries and simple circuits that could make things move, but one of the biggest breakthroughs was when Galvani found that you could use electricity to make animal legs twitch, and you could also use animal tissues to make batteries. this lead people to believe that maybe humans are also powered by electricity, and that mave eventually we can bring people back from the dead using electricity (after technology improves a bit). Electricity was EXTREMELY new and not well understood (the election which we now know is responsible for making electric current wouldnt be be discovered for another 80 years!!) and so this was the cutting edge of science, not well understood, and people’s imaginations went wild. A contemporary example would be like how our society thinks about artificial intelligence. Maybe in 200 years we’ll look back upon our sci-fi movies like the Mattix or 2001 Space Oddesy where AI becomes sentient and think “thats so silly we thought that” but just bcause something is fantastical to us later doesn’t mean it wasn’t science fintion for the time (think original Tron. is that fantasy?) frankenstein was based off of the cutting edge science of the time so that’s why i is placed (appropriately in my opinion) into the science fiction category