Why did babies get a startle reflex and not a sleep one?

Naptime with the swaddle:
Ginny: I'm so sleepy...
Me: Let's wrap you up and put you to bed.
Ginny: No, I'm fine! I don't want to nap.
*give her the pacifier and place my hand on her chest*
Ginny: Pacifier is fun. And woah, I'm tired. Actually, here's the pacifier back. My eyes aren't opening anymore. Zzzz...

Naptime without the swaddle:
Ginny: I'm so sleepy.
Me: Here is your crib.
Ginny: No, I'm fine, I don't want to nap!
Me: Umm... here's the pacifier?
*tries to give pacifier*
Ginny: You're doing it wrong. Let me hold it. And hold your hand. With both my hands. And wait, I can roll over. *rolls over* I don't want to be in the crib! *cries*
*tries to give pacifier*
Ginny: Hmm, pacifier. Let me hold it. No, wait, I'm on to you. I don't want to be here! *cries**gets head up, moves head to the other side* I don't like this side! *cries**gets head up, moves head to other side*
Me: Pacifier?
Ginny: I hate your stupid pacifier! And now my sheets are wet! *cries*
Me: They wouldn't be wet if you stopped crying on them. *moves baby*
Ginny: I don't want to nap! You can't make me! *cries*
Me: You sound completely exhausted...
Ginny: But I hate sleeping! *cries*
Me: Let's try your back.
Ginny: Hey, you moved me. Am I getting up?
*put hand on her chest and offer pacifier*
Ginny: Oh, that pacifier thing again. Nah, I want to roll over. Your hand is holding me back. I want to roll over! *cries and struggles*
*remove hand*
*rolls over* Man, I'm so tired. I'm so tired it makes me upset. *cries*
Me: You need to sleep, little girl.
Ginny: I don't know how to sleep! I'm so tired! *cries*
*pats back ineffectually. Offers pacifier*
Ginny: I don't want it! I'm too tired! *cries**blinks heavily**starts just fussing* This sucks! *cries hysterically, pushes head up*
*stands there ineffectually*
Ginny: I'm so tired I can't sleep. *cries exhaustedly**cries hysterically**cries exhaustedly**cries hysterically**cries exhaustedly*
Me: *cries*