Ultimately, 30% is just the commission that has become customary because Apple introduced it (in 2011, for in-app subscriptions to magazines, newspapers and music apps) and everyone else followed. Most major digital stores today apply the same fee:
Source: Analysis Group, “Apple’s App Store and Other Digital Marketplaces”
Business being business, everything boils down to agreements; and every agreement can be re-negotiated if either party believes it has the opportunity and sufficient leverage.
At the end of the day, Epic just wants to pay less to Apple for the privilege of hosting their game, under the assumption that finding the right compromise is mutually beneficial. And Apple being Apple, Epic knows very well that they would only react to a very hard push. Hence, all of this circus.