Why, because for hobby use, a mach3/4 board and license can be quit expensive, especially if you want to use a "modern" PC running a modern Windows version.
Mach, Linux, CSlab, Eding, etc are all capable systems and absolutely suitable for professional use. Nevertheless, I run Grbl-L, Grbl-L-Mega for years on my lathes and recently FluidNC on my mill and I don't miss anything.
I also have a Mach3 license and board but it just doesn't fit my needs.
Huub