But it works in games because, again, it's a different form of media with different ways to consume it. You can pause a book, you say. Well, you can pause a game, even in cutscenes. I've done it plenty of times. I literally cannot remember a cutscene that could not be skipped and/or paused and I play RPGs. And eye strain is not an issue with games. I have never met a gamer complaining of eye strain. You're more likely to meet one complaining of not having enough hours to play. Also, as someone who games and reads online a lot, trust me when I tell you eye strain is easier to get while reading on your phone. Which is how most people use sites like Tapas.
And speaking of the way in which people use Tapas, most people read while on break, or on the bus, or just in between tasks, meaning that they may not have long to read. If you give them a long chapter to read, they are more likely to lose interest and drop it, even if, as you say, you can "pause" a book. People lose interest in books very easily. It's why your first chapter is so important. Tapas puts these restrictions because they are beneficial to the users on both sides. Readers have quick bits they can look at during breaks and writers retain more readers if they post shorter chapters more often.
But going back to the game analogy, do you know why they can put long cutscenes you have to power through? Well, they're mostly before big fights, which means you've progressed far enough into the story that you're invested. Even if it annoys you, you're going to sit through it because you have to keep going or toss hours worth of gameplay. Books don't work that way. If you give someone a long chapter from the start, they're going to drop that book and not come back.
Ultimately, Tapas can't control what you do. They don't tell you what to write or how to write your story. What you post will be the same whether you post it in 15k characters at a time or 50k. You could post two or three story parts at the same time if that's what it takes to post a whole chapter, and it won't change the way your story is written. But it will change the way you build up your audience when you're posting so much they can't keep up.
No one can force you to post on Tapas if you don't want to, but you can't force them to serve your personal needs either. In this era where there's so much content available, you either adapt or get lost in the crowd.