There are multiple reasons why Yandere Simulator has been in development for a long time.
1. A game's development speed is directly proportional to its complexity. If a game is simple, it can be developed quickly. If a game is complex, it will take longer to develop. Yandere Simulator is a very complex type of game. It is a game with dozens of different activities, features, interactions, mechanics, and systems. Yandere Simulator is not the type of game that can be developed quickly. It is the type of game that takes a long time to be developed.
2. During a large portion of the game's development, I split my time and attention equally between development and producing content for YouTube. Because producing YouTube videos took up a lot of my time, I had less time for development, so development progressed slower than it otherwise would have.
3. Over the course of development, Yandere Simulator gradually transformed into a completely different type of game project than what it started as. It became the type of game that is regularly updated with new features and activities, rather than a game which is designed to offer a single linear experience. I explained this thoroughly in a video: https://www.youtube.com/watch?v=U1-f2RPUNTo
4. After making numerous personal sacrifices to devote as much of my time to development as possible, and spending several years working on the game for more than 12 hours a day, I became severely burned out, which affected my ability to be consistently productive.
5. After being character assassinated by dishonest clickbait YouTubers who spread false narratives and untrue accusations about me, I lost most of my financial support, lost many of the essential volunteers who were creating assets for the game, became extremely depressed, and lost most of my motivation to work on the game, which are all factors that reduce productivity and development speed.
Out of all of these factors, the false accusations have affected the game's development speed the most. Yandere Simulator probably would have been finished in 2019 if it weren't for all of the problems that dishonest clickbait YouTubers have brought into my life.
It usually doesn't take 8 hours for a man to walk across a football field. But, if there are extremely heavy weights tied to his arms and legs, then yeah, it might take him over 8 hours.
It usually doesn't take 8 years to develop a video game. But, if all of the above factors are simultaneously affecting the speed of the game's development, then yeah, it might take over 8 years.