There is a boundary between qualitatively different things A and B when they are continuous on a time axis. At the timing of this boundary, A ends and B begins. Therefore, "it is the end and the beginning" is logically correct, but since humans often recognize the familiar A and not so much the unfamiliar B, the sense that "A has ended" becomes stronger.
Related and more specific
You feel like things are complete when you've succeeded in taking the exam and getting into the school of your choice.
You think "it's over" when the tutorial is over and the main game is about to begin.
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