It doesn't, but nevertheless this is how it's done by convention. The exception to this rule is where you have an obvious lifting fuselage such as on a jet fighter with the leading edge strakes and tunnel between the nacelles or wide blended fuselage such as on the old Bernelli "lifting fuselage" designs. On things like that the lift contribution from the fuselage design needs to be estimated using some guessing or testing.

Wing area is easily calculated by the formula (tip chord + root chord) / 2 x span = area . And if done in inches then the wing loading per sq foot is calculated by weight in ounces x (area/144) = wing loading in oz/sq foot. No online calculators needed. Just some simple math.