> I'm not sure why the author limits themselves to parasites.

Parasites are distinct from most other selective pressures: they constantly adjust to whatever adaptations the host has come up with. Other pressures do not have this property - the desert doesn't get much drier because some plants adapted to the lack of water.

This also explains why worms have sex - even if they don't have parasites of their own (they most probably do), they need to adapt to their hosts' countermeasures.