why are the conditions for seed germination and plant growth different?

Answer 1

Mainly, the seed contains its own food source and is not going to carry on photosynthesis. Some seeds need light to germinate, but most seeds do not need light.

For the same reason, seeds need oxygen instead of carbon dioxide. They use the oxygen to get the energy from the stored food. Plants use more carbon dioxide than oxygen.

Both processes require water, so that's a similarity.