I think it's because the venturi effect results in the same pressure differential at any constant air velocity, regardless of the density of the air.
I don't know how to explain that, or use "laws" (Boyle's law etc) to justify it.
So, with altitude, and an unchanged venturi diameter, the same weight of fuel is being sucked into the venturi, though there is less air to combust this fuel. So the mixture needs to be leaned with altitude.
Please feel free to correct mistakes.