Epic hasn’t done it yet because it’s hard, computationally one of the most expensive rendering things you can do and Epic have to create a solution that scales probably at least to PS4. If you want GI now, use VXGI and scale it down and make modifications. Daniel Wright started working on an implementation of DFGI / Surfel GI a year or so ago, but had to put it on hold because of other priorities. Epic aren’t that huge in terms of staff who are able to develop that kind of tech.
Believe it or not, the requirement for DGI actually applies to a very small (but apparently vocal) percentage of the engines overall user base.