Explain Like I'm Five | Don't Panic!

All it takes to make one's sleep schedule go to s**t is staying up too long doing something interesting (reading, playing video games, partying), then the next morning the sleep deficit is so big, one would wake up at like 12 and then obviously wouldn't be sleepy at night, welcome to the cycle. Or opposite way, just stay in bed too long one day and it's the same result. On the other hand, even after forcing oneself to go to bed early, one might end up just lying awake for hours. Forcing oneself to get up early despite still being sleepy (basically sacrificing an entire day of productivity for the sake of a good rhythm) to fall asleep instantly at, say, 20 might work but doesn't have to (one can still be sleepy in the morning), and also it's not certain that in the evening one will be sleepy anyway (I call it the morning illusion of sleepiness and evening illusion of energy)...

TL;DR why does being (and staying!) an early bird require so much effort, while it's perfectly possible to become a night owl (and fall into the endless cycle) basically by accident?