Because sports is markedly different than the real world so that requires establishing different contexts for appropriate behavior. I can't go and randomly bodycheck a person in the grocery store like I can on a hockey rink. That's one of many reason I dislike your association of screaming at people in the course of normal life to be the same as it happening in the course of sports. There's literally a different set of societal rules that is applicable when you step onto the court / ice / field / whatever. Your last post echoed Leather's comments about context matters but you are questioning why I am bringing up context.
I think the college / pro thing is important because the stakes are objectively higher in college and pro sports. College can bit nit picked but most kids in the sports people actually follow (no, not fencing or sailing or one a smaller sport) are on some sort of scholarship / aid. A professional athlete's job is his / her sport. How a coach motivates his / her players should IMO be different than how a high school coach motivates his / her team. Sometimes that involves rainbows and butterflies and sometimes that involves getting in the person's face. Again, it shouldn't be the only tactic but I firmly believe it should be a tactic available to the coach to deploy as they see fit.
You think people getting up in your face and yelling at you is a line to never be crossed in any situation. Which is fine. But it's also a completely arbitrary definition based on your personal communication preferences informed by non-sports interactions.