Underground rapper vs Idol rapper (Genuine question)

Hello! Please excuse my English as I am not a native speaker. I'm curious with this thing. Overall, we all knew in terms of rapping, underground rapper do better than idol rapper. There's a show called SMTM & I didn't watch full episodes but there's a lot clips of it in Youtube. So, here's my question..

Based on SMTM clips, I saw not all underground rapper were that great in rapping. Yes, some were so good. But the rest of them? Their skills can match idol rapper. But damn, they really look down on idol rapper a lot during the show. Why tho? Is it really pure discrimination? Or underground rapper has some 'ego' for being called as 'underground rapper'? Making them different from 'idol rapper'. Are this 'underground rapper' really the real rapper?

I know that idol rapper's skill are nothing compared to underground rapper. Even though, we still have great idol rapper that used to be underground rapper such as BTS rapline, Straykids Changbin, Bang Yongguk, Zico etc. We even have G-Dragon. All of them are idol rapper now. My question is why tho?

P/S: Pls don't throw any hate towards me. Im just asking genuine question & pls reply nicely. TQ