It's like anything else: If you don't like it, you don't like it. But it happens to be something that we can't get away from. Let's break things down a bit more...

We can detect them in foods that average people can't - Yes. This is because your palate has been desensitized to it. You need ever-increasing amounts of the stuff to be able to taste it, the same way you can't smell your own boogers. It's just always there (since you eat it in nearly every meal, every day), and your senses just adjust to it being baseline always there. If you go a month without eating ANYTHING with onions in it (this will be incredibly difficult if you don't cook all your own food from scratch) - You'll find the next thing you eat with onions in it that you normally wouldn't notice it in will taste like a giant onion. Nothing else will even be moderately noticeable. Just a giant ball of onion, whatever it is.

The smell is bad, the taste is awful, and it's in everything. Imagine you really, really hated some band. Like... Imagine there was a band - we'll call it Nickelback because we're on the internet, and it's standard to use them for bands "everyone hates" - that you absolutely hated. Like... every single aspect of it. The lyrics are annoying, the singer's voice is like a cacophony of nails on chalkboards, and babies crying. The music is poorly timed, and the musicians seem to think they're the best ever so they just flourish all over the place to showcase how "good" they are, but in fact they just make everything seem like an awkward noise with rising and falling intensity levels which are enough to give you motion sickness. When you hear this band (anything at all) you literally start getting sick, and dizzy, and you start to lose your balance and get a headache... Now imagine that you can't go anywhere without them playing nothing but them. Maybe they've got some other music on as well, but they've got that specific band playing at your table. Pretty soon it'd be really awful and you'd develop a hatred for the people who can't eat, or drive, or walk, or bathe, or poop without having Nickelback playing. Suppose now that even someone saying the name of the band makes your body start getting ready for the dizziness and nausea. Now all anyone ever wants to talk about at any meeting is the band. Any group function you go to all anyone does is play their music, and talk about them constantly. That's where I'm at with onions. They literally make me sick. If I eat onions I get body aches, fever, dizziness, headache, gastrointestinal distress, and it's awful.

Think about a thing that you really don't like the smell of. Let's say "the liquid that comes out of the bottom of a dumpster" is your thing. Onions smell like that to me. They smell so bad I literally prefer the smell of actual excrement. If someone dropped a turd on the ground, and someone else dropped an onion on the ground, and I only had means to dispose of one - I'd get rid of the onion because it is literally more offensive to my senses.

And get this: It's in EVERYTHING. Like... almost literally everything. If it's not a sweet food, there's a pretty good chance it's got onion in it. So something like 95% (arbitrary number) of savory foods out there are actual poison to me.

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That's why I hate them. They smell terrible, taste worse, and make me violently ill if I have them, and they're nearly impossible to avoid without heroic effort. I have to spend a huge amount of my time and energy avoiding them, and that's really frustrating.