Because it comes with the freedom to used for any purpose in its core goals / social contract [1], including commercial purposes:
> No Discrimination Against Fields of Endeavor
>
> The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.
> No Discrimination Against Fields of Endeavor > > The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.
See also [2]
[1] No Discrimination Against Fields of Endeavor
[2] https://www.gnu.org/philosophy/selling.html