I only reply because you said it's a genuine question.
1. This can be said for any fetish (foot fetish for example). Sexuality, in general, apart from it being a means of reproduction, is a weird thing.
2. A goblin is a creature, usually weak, usually kinda cunning, with a monstrous appearance, and - in hentai - with addictive cum. So it's kinda like being an average unattractive male with a sex-related superpower.
3. A goblin's purpose is oftenly shown to be their reproduction. Therefore, the goblin's goal is pretty much to impregnate the female humans. This, paired with point 2, makes it speak directly with our base sexual instinct - which is to reproduce.
4. Goblin societies aren't usually shown to have children, or to educate children. Sometimes they are shown as babies, but in their society it's highly probable that their kids are left to fend for themselves, therefore aren't the responsibility of the parents (or are, but for a limited amount of time). For humans, this responsibility lasts for about 18 years (more, in some cases). I am assuming that this could also play a role, paired with point 3, where by playing the role of a goblin, you are free to explore those base sexual instincts, as then, the responsibility related with a pregnancy "feels" miniscule.
Obviously I've no idea if what I said is correct, but this is a possible explanation that I thought of.