Question

why the air is not be seen by us

Answer 1

because the air contains very small particals of atoms and moleculs

Answer 2

$\bf\huge\color{red}\mathbb{Hello\:User....}$

We can't see air all because our retina is not sensitive towards its constituent particles.

Our retina have evolved that are sensitive towards the very wavelengths of light that passes through it and if the particles of air were huge enough to reflect the beam of light then we could definately see Air.

$\bf\huge\color{red}\mathbb{:)}$