There's several reasons why someone would want to eat disgusting food.
Poor people will eat whatever they can afford.
No, really. Ever notice that most of the disgusting foods you've named are part of the regular diet of people in depressed areas? If you can't afford much, you won't waste any vaguely edible animal part you can get. If all you have is a sheep, and you have to feed a family of sixteen, then you're gonna damn well going to eat every single last bit of that sheep.
Hence, haggis. Or pig's feet. Back home, we don't make gravy from chicken or turkey innards - we make new dishes. I particularly like balun-balunan - chicken gizzard. We used to buy this by the kilo...
Also, if a locust swarm flies in and destroys your crop, what do you do? You have fried locust for dinner. You might at least get a little revenge.
I've eaten rat, taken straight from the rice fields. Country rats are pretty clean, as opposed to city rats, which carry sewer stench (you can smell it on the meat).
Of course, rice farmers only eat rat when the harvest is lean, and you can't afford chicken.
Maggots, termites, and other insects are good protein sources. There's nothing quite like the taste of a fat, juicy grub, roasting on the end of a stick. (I generally spit out the head and the hard mouthparts, but some like crunching it).
Bet you can't eat that.
Never underestimate the power of the dare. Especially when you and your friends are really, really drunk.
Anything can become fair game. Dogs. The aforementioned rats. Live goldfish. A Filipino delicacy is bayawak, or monitor lizard. Think Komodo dragon, but smaller, about a couple of feet long. Of course it tastes like chicken - that's what the lizards mostly eat.
I've even watched an uncle pick up a pink, newborn baby mouse, dip it, still wriggling, in vinegar, and pop it in his mouth.
It's like herbal Viagra y'know?
And of course, last but not least, people who eat animal parts because they believe it'll make 'em stronger/smarter/better in bed.
Tiger claws, powdered rhino horn. Shark fin soup. Most of these stem from ancient Chinese practices, and are consequently more popular around the Asian side of the Pacific Rim. This seems to be waning, especially with the ready availability of Pfizer's wonder drug.
Filipino equivalents are many - including balut. You could also try what is discreetly labeled in roadside kitchens as "Soup No. 5". This consists of stew made from bull genitalia - although most times, this qualifies as "dare" food, it is also believed to enhance sexual prowess. Betcha can't eat just one. :-)
So remember, every time you shy away from eating something disgusting - there are kids on the other side of the planet who are probably sick and tired of eating it every day.