Why do complementary nucleotides across the double-stranded dna bond together using hydrogen bonds rather than covalent bonds?

Hydrogen bonds are much weaker than covalent bond because of its bond is formed through fundamentally electrostatic interactions.

Hydrogen bonds happen due to dipole-dipole attraction while covalent bonds happen when valence electrons are shared.

Reasons why complementary nucleotides across the double-stranded dna bond together using hydrogen bonds rather than covalent bonds are (1) the distance between two strands, (2) size of the bases, (3) geometry of each base.