Question

why do frogs lay hundreds of eggs?

Answer 1

because many of the eggs die straight away some of the tadpoles and eggs are eaten so only a few frogs reach the reproductive age

Answer 2

Most frogs need to lay their eggs in water . Each female frog lay thousand of eggs at a time in slimy masses . The eggs hatch into tadpole ,which look different from frog . The frogs breathe gills instead of lungs.
I think this is reason.