China was never a "mother" figure. Certain empires in China became powerful enough that they inspired neighboring kingdoms to adopt and adapt certain social mores, cultural practices, and technological innovations. For example, during the Nara period, the Japanese tried to copy and learn from the Chinese as much as they could.

China referred to itself as the Middle Kingdom -- the center of knowledge, innovation, and culture. It measured the kingdoms in its periphery according to their willingness to accept Chinese culture and influence. Japan and Korea were referred to as "younger brothers", willing to learn from their "elder brother" China. (The terms stem from the Confucian "Five Relationships".)

This feeling was not always mutual, as the kingdoms in Japanese and Korean history often were not very welcoming of China's condescending approach to international relations. At best they tolerated China's friendly meddling. At worse they would go to war to re-assert independence. Take note that this would be the case over the centuries, especially as China's empires would wax and wane regularly.

Now: why do they not have this friendly, brotherly relationship anymore? Political reasons. China's power and influence deteriorated during the Qing Dynasty. Japan -- a country China has traditionally viewed as its younger brother -- had overtaken it in terms of military might. The first Sino-Japanese War was a huge blow to China's ego.

As such, Japan and Korea may look at China as a source of cultural influence (something completely undeniable), but both countries already have significant influence and power of their own. They had to tolerate China in the past (notably during high peaks like the Han Dynasty and Tang Dynasty), but to say that they "drifted apart" is not accurate. The motherly relationship was never there in the first place.

There is so much more to say about the dynamics in Northeast Asia. Both Japan and China have used history in the region to legitimize aggression, often with Korea on the receiving end. Asserting independent development is a big deal in Northeast Asia, particularly for Koreans. This is the reason why you should never confuse a Korean person with someone Japanese. It's the sort of insult that spans centuries of conflict.

A final note, especially on the matter of language: Mandarin is completely different from Korean and Japanese. Korean and Japanese languages are more likely rooted in Altaic. Mandarin is non-alphabetical, unlike Korean and Japanese. Finally, what Japan and Korea copied from China was the written language, with some borrowed words. Just a clarification.

If you take "China" as something like "(Christian) Roman Empire" then I think you'll figure out why. Most European nations are influence by Christian Roman civilization. Their languages are affected by Latin, share some similar (but localized) festivals, and lots of regimes claim to be a "successor of Roman empire" which means this regime is more civilized and powerful than others. However, would Europeans look at Italy as a "mother nation"? I don't think so.

I don't want to overgeneralize, so I will just give 1 anecdote.

I used to go to a Korean American dentist. I don't know if he was just trying to be nice, but after he found out that I'm Chinese, he told me that SOME Koreans consider China to be "mother country".

That's just what I heard from 1 person.

I have also heard from some that some Korean elite families consider learning Chinese to be a must for being elite.

I'm sure there are some counter-examples.

But I don't think there are similar examples in Japan.

Again, not meant to generalize. Just what I have heard.