I made a Mac app a few weeks ago that got to the front page here on HN[1]. It isn't notarized by Apple since I don't want to pay Apple just so I can give my work away for free that fixes something that shouldn't be an issue in their OS in the first place. When users run the app for the first time, they get a warning[2]. For that kind of popup to happen when anyone tries to use anything by Epic would almost certainly dissuade new users a bit. Currently you can run unsigned code on macOS if you disable some security options in preferences but soon that won't be possible[3]. It's already the case that unsigned code can't run on iOS, so for Epic to develop an engine on iOS they must be able to sign their IPA files so they can actually be installed on a testing iOS device which can only be done through Apple.
1. https://github.com/ther0n/UnnaturalScrollWheels/
2. https://user-images.githubusercontent.com/5874301/88485840-2...
3. https://www.reddit.com/r/apple/comments/icyif5/apple_silicon...