Here's why:

* Mass is the amount of matter in an object. It's a fundamental property that doesn't change based on location or gravity.

* Weight, on the other hand, is the force of gravity acting on an object's mass. It can vary depending on the gravitational field.

* Laboratory balances are designed to compare the unknown mass of an object with a known mass. They work by measuring the force required to balance the object against a known mass. Since the force is directly proportional to the mass, the balance effectively measures the mass.

While it's true that balances work by measuring forces, their primary function is to provide a reliable measurement of the object's mass, which is the fundamental quantity of interest in most laboratory settings.