Here's why:

* Electronegativity difference: Silicon (Si) and hydrogen (H) have similar electronegativity values (Si: 1.90, H: 2.20). The difference in electronegativity is very small, meaning the electrons in the Si-H bonds are shared almost equally between the two atoms.

* Symmetry: The tetrahedral geometry of SiH4 ensures that the bond dipoles (which are very small due to the electronegativity difference) cancel each other out.

Therefore, even though each Si-H bond is technically slightly polar, the overall molecule is nonpolar because of the symmetry and minimal polarity of the individual bonds.