The concept between Gigantamax is different to the one behind Mega Evolution so why would they reuse the forms.prior to dynamax, there was no reason to think Game Freak would toss them aside. as soon as gigantamaxing was revealed, it wasn't too crazy to believe that Megas would have been repurposed. as I said, making new designs and not using them was a weird concept and yet they did it.
I get wanting to have a different battle mechanic, but the asset usage was just weird there
Sorry but this is an odd complaint