Because everyone else basically said "the new conditions are bad, I won't risk my business", but in the particular case of Epic they're banned from Apple's walled garden, so even the terrible terms are better than nothing.
Apple has said as much for the main developer account in the past.
It is highly unlikely Epic would publish anything in the App Store, however, even ignoring the bad blood.
Part of operating an App Marketplace is that you are agreeing to the EU rules which include a core technology fee. So even an app with no in-app purchasing on the Apple App Store would cost them a substantial amount to publish.
Apple overplayed their anti-competitive card, so now they'll be permanently scrutinized.