Answer
Time dilation explains why so many muons are detected at sea level.
Work Step by Step
The muon is moving fast enough that the time dilation factor, $\gamma$, is substantial. From our perspective on Earth, the muon’s internal "clock" is running slower than Earth clocks, leaving enough time to reach the ground.
In the muon’s own frame of reference, it lives the normal lifetime, but the height of the atmosphere is contracted by a factor $\gamma$, so it has enough time to arrive at sea level.
