I somehow agree with the analogy of OP, it looks the same with ivy league. Graduating from a prestigious university can build your first step more stable. It will give you a big advantage in your first year, just like how applying for a job is the same as debuting.

For example, both graduates with the same GPA from the same program, and what if they applied for the same job? Somehow, there is a factor that an employer will accept the one who graduates from a prestigious university. Why? Because a prestigious university has different standards in terms of quality of education.

Same with fans, artists from Big3 tends to have more supporters because they quite have the reputation that the company's training has a different standard from others.

But just like in the corporation world, even though you graduate from a prestigious university and landed a great job, it will not guarantee your success. Because as the time comes by, they still need to improve the skillset, and not just rely on being a graduate from a prestigious univ. Just like how these artists from Big3 need to prove why they are from the Big3.

And not all successful CEO's are graduate from a prestigious one. This is what happened with BTS.