Fsbuild-2 Flight Planner

Why does that happen where the plane takes off at the supposed time due to other planes in front of it, why does the airport let that happen? Its annoying when the plane is stuck on the runway due to other planes and how often does it really happen? and can the plane even be stuck on the runway for hours due to other planes. Why cant they do something about that of other planes in front of their plane and what airports does that happen to most in?
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