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Because it is far more satisfying to take down someone who thinks they're hot shit (DICKS, Silver, Barry, Wally to a lesser extent) than it is to take down those 'lol let's all have fun' shitheads (Bianca, Hau, Tierno, Trevor, Shauna). To say nothing about the completely indifferent crybabies who take it up the ass without complaints (Brendan, May, Hugh). The absolute worst are the ones who bitch and moan about losing when they're not even that good to begin with (Cheren, Calem, Serena).
It's not the goals, it's the investment the world puts you in. DICKS, for example, keeps thinking he's on top of the world because his grandfather is a well-known scientist and he was "chosen" to complete the Pokedex (alongside you, of course). He's always one step ahead of you at every point in your adventure, and always talks down to you at every opportunity. This is the kind of guy that you'd actually want to throw a punch right into his smug little face because of his attitude, and you get that satisfaction when you best him by dethroning him in the Champion's room - something that your entire journey has been building up to.
Then you have people like Hugh who are just "I gotta find muh purrloin, but hey let's spar/team up for a bit". You have no investment because you're not aligned with his story or goals, and therefore get no satisfaction from beating him. He's not trying to become the champion. He's not trying to catch 'em all. Hell, he doesn't even talk down at you, much less even think of you. He's on his own little quest that you're not involved in - you just happened to show up where he does.
Then you have crybabies like Calem/Serena who beat themselves up after every loss. It's like kicking a child while he's on the floor crying. It's not fun unless you're the sadistic kind of person. It'd be different if they weren't total trash as trainers, but that's how it is.
