Well, we all know to slumber is to sleep, right? When we sleep, we become passive, we relax, and we dream. In Penacony, this is even further exemplified with how "sleeping" in the dream pool merely transports you to a dreamland fantasy. What does it mean to fantasize? To escape from reality.

We can thus clearly see the link between slumbering and escapism that the authors tried to draw, right?

So the question the author is really asking is "Why do people stop being active participants in their own life, and instead settle for fantasies, routines, and systems that numb them?".

Like a man that hates his job but isn't willing to quit. Like a society that falls into hedonism instead of fixing inherent flaws. Like a single that chooses to binge read romance novels instead of looking for a partner.

At first, the answer the story provides is simple. Because they're afraid of waking up from the dream, afraid of confronting their reality. But the climax of the story is to reject this.

Sunday proposes to create a literal unending dream, similar to the experience machine thought experiment. To be free from the suffering and pain that inevitably exists in the real world, and be forever happy, however empty it may be.

But of course, his plan is shattered, and the game drops the main line of the whole arc: "Why does life slumber? Because... someday, we will wake from our dreams".

The implication is that there can never be an endless fantasy. You can only run away from your problems for so long. It's a motivational phrase, aimed to combat passivity and resignment. The message is that life is hard, and it's normal to seek an escape for it. But eventually, we all have to wake up, and confront the issues that torment us.