There's just a lot of inherent risk to sharing your big roll-out reveal in the same week as your primary competitor. 2013 Kinda showed that when both Xbox and PlayStation share details so close to each other, the horse-race media frame becomes inevitable. Sure, it'll always happen, even if the console releases are staggered a year apart (see PS2 vs. Xbox/GC or X360 vs. PS3), but at least then you have a year to build a library. Now they're releasing in the same year again, I think both parties are seeing the risk-reward proposition and deeming it not worth taking.
Sony won with ease last time around, but amongst core gamers/early adopters I think there's a brewing desire for an Xbox comeback story. Does that demographic matter much? Not really in the long run, but for initial buzz it's nice to get them on your side.
Microsoft is pretty effectively signaling they're in this for the long haul, with a focus on software going forward. They're building momentum, kinda like how the PS3 did towards the end of that generation. PS3 dropped prices hard, doubled down on software and just tried to get into as many homes as possible; which is how they ultimately overtook X360 sales late in that generation, and carried great momentum into the current gen. Right now that's basically mirrored in the Xbox One S/All Digital strategy; sell them for 175$ with 3 games included and GamePass as your trojan horse.
I don't know how much Sony could gain from E3; Microsoft could undercut them somewhere (be it price, launch line up, maybe they'll make Xbox Live free) and then you're left scrambling. Sony already ceded E3 to Xbox last year, so it feels like less like they're avoiding a direct confrontation and more like a continuation of a new business strategy.
They can skip (or undercut!) E3 without losing too much face, and their 100 million consumer base will likely be largely receptive to their future marketing endeavours anyway. It's Microsoft who need to prove themselves on the big stage - if MS goes all out, Sony just risks looking weak. Better to launch on their own terms, business-wise.