I am wondering why the Present Moment just happens to exist everywhere at the same time.
I understand your point about simultaneity, but what I'm trying to get at is, even though two observers would not agree when an event happened, their "nows" would always be the same. It seems to me "now" is independent of relative time frames.
Two observers at different distances from a super nova explosion would not agree when that event happened according to their clocks, but they would always agree on when "now" is occurring. This would also be true in time dilation. One observer's clock may be ticking at a different rate relative to another observer's clock, but for both observers, "now" would always occur at identical moments in their relative time frames, irrespective of what their clocks say, and irrespective of their relatively different rates of elapsing time.
Here I add Disclaimer.