Why is a mile exactly 5,280 feet?

In ancient Rome land distances were measured in paces, one pace
being defined as two steps, or approximately 5 feet. The largest
unit of land measure used by the Romans was the "milia passuum,"
which is Latin for 'a thousand paces.' As one pace equaled 5
feet, a 'milia passuum' was 5,000 feet.

Once a part of the Roman Empire, Great Britain adopted the milia
passuum but abbreviated it to "mile." The British also used
another unit of land measure called the furlong, which was
defined as 660 feet, which was the approximate length of a plowed
furrow in an ordinary field. The furlong was used extensively in
surveying, and when it became apparent the mile would also be
used in surveying, it was decided the mile should be a whole
number of furlongs. Since the mile was 5,000 feet and the
furlong was 660 feet, it was decided to change the mile to
exactly 8 furlongs, or 5,280 feet.

In 1575, Queen Elizabeth I passed a law making the mile exactly
this number.