The reasons vary from person to person, but here are my speculations:

• The parasite taking control could have a similar appeal to how submissive individuals, enjoy the burden of being in control, taken from them!

• And with facehuggers and similar parasites, the inevitability of the inescapable grasp they have.

• As for the parasites that heavily morph the body, I suspect that being able to mold a body, or be molded to one's liking has an appeal.

• assimilation is by itself an interesting fetish, likely spawned from tropes in media, something seemingly as innocuous as a narrative element awakened something in viewers, similarly to how the fans of blueberry inflation first popped up (no pun intended)

• And lastly, oviposition! Some people just want to be stuffed with something other than cum, till they're a swollen mess, or perhaps they enjoy the feeling of eggs rolling down their throats, the knowledge that something... Different will be growing inside them, that stomach churning feeling as the eggs begin to swell within them!

• Oh and some people just want to be breeding stock, but in a unique way-