What else do these all have in common?
They all have art styles that are purposefully designed to be really cheap and easy to make. The same choices that helped these studios crank out hours upon hours of formulaic content also helps the rule 34 artists.
The short answer is: the industrial revolution.
Prior to the industrial revolution, labor was mostly agricultural, which meant hours varied based on the season, weather, daylight, and vibes. The rise of factories led to factory owner attempts to control their workers more. Better technology for mass-producing clocks meant factories had them and used them against workers, including having clocks that purposefully tracked time incorrectly in order to make workers work longer. it was just one more tool for capitalists to use to exploit labor. The concept of a “company clock” or being “on the clock” came from this. Prior to this, workers were paid for production, not time.
Historia Civilis has a great video on it. Which is a very strange outlier as there is nothing else remotely like this on the rest of the channel.