I don't know. The current status quo makes it very easy to "glue" different layers of code together assuming certain contracts, and allow those contracts to be extensible later on (handling optional extensions if one wants).
Now, "easy glue" has a number of traps, but opting into the status quo today would make my life a lot harder. For example there are times where I want to say "given this data structure, take the relevant subset and create an object from it" and Moose is currently very good at doing that. Moreover if Moo were to adopt this behavior too, I would have to write additional constructors for handling that case on every class.
As I see it, Moose has a large number of ways of ensuring a flexible yet robust software contract besides this. You have type constraints, required attributes, and more. Since these are remarkably helpful I don't see why this should be the default behavior.