>DDLG. …Why are you into father/5 year old daughter incest roleplay?

 You’d have to ask someone who was into that.  

But if I was into that I’d probably say it was because my adult partners and I were into it.

Into it the way adults can be into roleplaying other objectively silly but subjectively arousing themes like pirates, gangsters, movie stars, millionaires, grossly-stereotyped ethnicities, soldiers, nobility and “French maids,” doctors and “naughty” nurses, cable guys or pool boys and bored housewives, bosses and secretaries, virgins and rakes, strangers-in-a-bar, porn stars, “hookers and johns,” prisoners and guards, cops and criminals, vampires or werewolves, imprisoned princesses, unicorns or other furry animals, characters from 50 Shades of Grey, characters from Mad Men, characters from My Little Pony (hey, I’m not gonna judge), or clowns (still not judging), or husband and wife (when they’re not) or having an affair (when they’re monogamous together), and on and on and on.

Or you might ask vanilla people why they’re into roleplaying repressed Victorian-era missionaries.  (Worth mentioning that before the Victorians it was Puritan doctrine that husbands and wives would be unfailingly lusty for each other, so practitioners can’t fall back on missionary sex being “normal” or “religious.”)

Review each item in a list of “naughty” Halloween costumes and ask yourself why that?!?!

The short answer to any of the above is that for many adults relationships are social as well as psychological or physical.  And so adults find ways of bringing social tropes and metaphors into their interpersonal relationships.  They also often seek out other adults who share not only compatible sets of body parts but compatible social metaphors.  If you both happen to have strong social associations with Spongebob and Patrick, then you may enjoy bringing that dynamic into bed together.  And no, as long as you and your partner were adults who were exercising affirmative, competent consent it would be silly to judge that either.

Speaking for myself I’m not into roleplaying being sexual as a father, or being sexual with a minor offspring.  D/Lg kink ≠ incest kink.  Instead it’s just fun and erotic to intentionally employ the social cliches and stereotypes of caregiving as an adult with an adult partner.  

One gives a pacifier to an infant to… well… pacify them.  One gives or receives a pacifier to an adult sexual partner to signify an ephemeral, consensual alteration of relative power.  That’s… pretty different from incest.

Ok, the shortest possible answer is Daddy ≠ dad!

Hope that helps answer your question.