1/2 because the (1/2rho*V^2) component of the lift formula is the mathematic indefinite integral of rho*V .
Of course, I was joking about the 3/2!!
It is also mathematically convenient for it to be 1/2 in the lift equation because the quantity (1/2rho*V^2) can be calculated directly from Bernoulli's equation if you know the pressure.
All of this theory applies only to inviscid flows however, once you have viscosity in the frame it's a whole different (and more complex) ball game!
Also, applying Bernoulli's equation to wing cross-sections can be quite tricky and requires a mathematical concept known as a conformal transformation to make the geometry (if not the algebra) somewhat easier to deal with!
B