why a buffer considered a good standard for the calibration pH meters?

Answer 1

Whenever you need to calibrate an instrument you need to use stable chemicals of known concentration with as high accuracy as possible.
Buffers have the ability to keep their pH constant when small quantities of acid or base are added to them and also their pH is not affected by small dilutions.Thus the buffer solution will have a constant known value which will not be affected by things like atmospheric CO2 solvation, drops of water on the electrode ,etc.

In two words it's a more robust system than a simple solution.