The main reasons the atmospheric oxygen remains constant are:
The sheer quantity of it. Not even a large forest fire will
measurably deplete it. Most of it is fossil oxygen created many
millions of years ago by photosynthetic organisms.
It is constantly being replenished by photosynthetic organisms,
mainly in the ocean.
Measurements of the percentage of oxygen in the atmosphere are a
modern phenomenon. There are no accurate measurements of atmospheric
oxygen in previous geological periods, but we know the percentage
has not always been 21 percent.