Planck units are often referred to as "natural" because they are derived solely from fundamental physical constants without any arbitrary scaling factors. They are defined in terms of the fundamental constants of nature, such as the speed of light (c), the gravitational constant (G), and Planck's constant (h), which are believed to have universal significance and not depend on any particular system of units.
Although Planck units involve the use of SI units, such as meters, kilograms, and seconds, the values of these units in the context of Planck units are determined solely by the fundamental constants, making them independent of any specific system of measurement. In this sense, they are considered "natural" because they are based on properties inherent to the laws of physics rather than being defined arbitrarily.
By setting certain fundamental constants to unity, the Planck units effectively provide a system of natural units where the magnitudes of physical quantities are expressed in terms of ratios or multiples of these fundamental constants. This allows physicists to express physical theories in a more fundamental and elegant manner, free from the particularities of any specific unit system.