Actually this is something I've been wondering quite a lot lately.
FSX really is something of a bygone age, stuck in technology which does it no good (the 32 bit OOM problems you just mentioned, lack of optimization for newer GPUs and lack of real multi core support).
I was actually surprised to discover Prepar3d, I thought someone finally had made a new flight sim for the new age, but it's nothing more than a spruced up FSX. I fear it suffers from the same limitations, and some people say it's even slower; however to be fair, it does look a little prettier, and the mod community is starting to turn its way.
Not aware of that other sim you mention, but I've caught myself thinking several times "it's about time someone does a civilian sim with the quality of DCS or War Thunder".
If I had to guess, the effort of modeling the whole world, is what's keeping a potential developer from undertaking such a great project. If it doesn't make money, it's hard to do, however nowadays we have platforms to crowd fund these exact kind of projects.
Here's to hoping someone will pick up the torch.
