Its because 1.0 isnt supposed to be "The game is complete" its "This is the first version of the game that we are releasing."
Lots of "early access" games are clearly not complete but make their first released version 1.0 because its more confusing and hard to keep track of to try to use anything less than that to keep track of versions. I mean really think about it
"Version 0.001 we released the game!"
"Version 0.001.0.1 we added A B and C to the game!"
Its at best silly and nonsensical and more realistically confusing and convoluted. Version numbers aren't meant to be a metric of how 'finished' a game is, its meant to be an organization of how many updates there have been.