But I don’t suppose anyone has yet found a reason why expansion and density should constrain each other?
Given the incredible improbability ( a “0.00000000001%
David Taylor
22

One explanation is that they are not linked and we are only subject to survivorship bias: countless of big bangs have produced (and still are producing) countless universes all with different parameters, and in most of the universes there is no one to ask these questions.