You're comparing between different sample pools, which matters when we're talking about probability adjacent stuff. We're not asking "from this large pool of people at an airport, who is likely to receive additional scrutiny?" Because of this, your comment about how you've seen people of all backgrounds get scanned isn't relevant to OP's point.
The scope we're looking at is the pool of experiences across one person's trips. Imagine if it was every time that you got stopped for additional checks at an airport, even when you couldn't see any mistakes that you had made. If you get checked because your keys triggered the sensors, then that's a mistake that you can learn from, but consider how it would feel if you meticulously complied with everything you were meant to do, but were still consistently pulled aside for additional checks.
I know that on the internet, you never know whether someone is being hyperbolic, or straight up spinning a yarn, but try to take OP on faith here and consider how dismissive your comment comes across. I don't know OP's particular circumstances, but I have previously made a comment similar to yours to a friend, who called me out on being an asshole. Back then, I was oblivious to the reality of these things.
My friend explained that the first time they were pulled aside for additional checks, they opted to believe that it was just a random thing. The second time, they felt more uneasy, but actively resisted the "victim mentality" (their words). By the 20th time, they had come to expect it as inevitable, and that no change to how they packed, or what they wore would change things. They desperately wanted to believe that they weren't being targeted for additional searches, but after a certain point, it becomes impossible to believe that these things are random.