Question

why has macbeth been adapted so many times over the past 400 years

Answer 1

Macbeth has been adapted as much as how Shakespeare's works were adapted as well. It is because Macbeth has elements in it, like the dark rise of power and ambitions that tend to go over the edge, that relates with many modern-day situations. 

Answer 2

Audiences find the main themes of ambition and desire relevant regardless of the time period