Why do we use the Planck length? We are not able to quantize spacetime and the theory of  loop quantum gravity has been proved wrong but we still use the Planck length in astrophysics. Why?
First off, you're much more likely to see parsecs, light years, or astronomical units than Planck lengths in astrophysics.

However, for those who like to work in a system of natural units, the Planck length is the obvious choice for the unit of length. This choice has nothing to do with quantizing spacetime. It instead results from a desire to make conversion factors such as the speed of light $c$, the reduced Planck constant $\hbar$, the Boltzmann constant $k_b$, and the gravitational constant $G$ all have a numerical value of one or some other nice number such as $
\frac{1}{4\pi}$.

One way to look at this: Compare the conventional units used in the US and a few other backward countries. These countries have to use $F=kma$ rather than the $F=ma$ used in the metric system. The $k$ is a conversion factor that results from having those conventional units being an inherently inconsistent system. From a modern perspective, force is the product of mass and acceleration; no conversion should be needed.

That said, that the mass-energy equivalence equation expressed in metric units is $E^2=\bigl(m c^2\bigr)^2 + \bigl(pc\bigr)^2$ is a sign that the metric system, too, is inconsistent. With natural units, this expression simplifies to $E^2=m^2+p^2$. Energy, mass, and momentum all have the same units; no conversion is needed.