Here's why:

* Gametophytes are the haploid generation in the plant life cycle. They produce gametes (sperm and egg) through mitosis.

* Seeds are produced by the diploid sporophyte generation. They contain a fertilized egg (zygote) and a food supply.

* Pollen is produced by the male reproductive structures of the sporophyte generation. It contains the male gametes (sperm).

In summary: Gametophytes produce gametes, while sporophytes produce seeds and pollen.