Because it's practical to ignore the gravitational acceleration in everyday activities.
The weight of an object is simply a measure of the force the object has as a result of gravity.
The mass of an object can be though of as the amount of matter that respective object has.
The weight of an object is actually a vector quantity. The magnitude of that vector is the product between the mass of the object and the gravitational acceleration,
#W = m * g#
As you can see, the only difference between the mass of an object and the magnitude of its weight is the gravitational acceleration,
This means that for most everyday activities and situations, the difference between the weights of two objects will actually be equivalent to the difference between their two masses.
Let's say that you put an object on a scale and read its weight to be
In this case, the word "weight" is used correctly, but the value that's being read is actually interpreted as mass, that's why it uses kilograms instead of Newtons.
If the scale were to give you the weight in Newtons, then you would have to get the mass of the object by dividing that weight by the value of
#m = W/g = "9.81 N"/(9.81"m"/"s"^2) = "1 kg"#
Now, although this is the correct approach, it is not very practical for everyday situations because the value of
Moreover, your body is so accustomed to the value of
So, as a conclusion, everyday activities do not require a distinction between weight and mass, which is why our scales measure weight but use the units of mass to express it.