The second reason. Companies that are worse at getting people to self-exploit tend to do worse financially, and thus tend to disappear. Of course, if too many people quit or burn out, then the company will go bust, too. The optimum will be just one step short of that. And whenever someone starts a new company, they will look at the existing successful companies for how things are done.
There is no evil master plan. The processes that lead to self-exploitation will sound reasonable and well-intended. (If this wasn't the case, people would refuse to adapt it. Most people don't like exploitation when they see it.)
For example, in most of Europe there is a law that requires employees to record their working hours. This law was created to prevent (unpaid) overtime, and it does that. Now company X implements this by making you write down how many hours each day you worked towards which task. Doing this every day makes you think where you put this one-hour chat you had with a co-worker. It was nice, but which task did it contribute to? (It didn't...?) If you bother to ask, everybody will actually encourage you to have those talks, that it is even in the interest of healthy company culture, and remind you that maybe it's part of the paid break (you didn't forget you have that, did you? it was never anyone's intention that you skip your break). Nobody will be responsible for nudging people towards efficiency. It's all the fault of the individual who feels pressured into efficiency. It was never anyone's intention to prevent you from having those occasional nice chats, and nobody will stop you if you keep doing it.
Still, every day you get to think about how long it took and which task it belongs to, and it feels a bit like lying to just add the time to a random task. This kind of habit can shape your thinking.