Having GUI wallet for windows or not, at the current period, does not matter.
It matters a lot. Where can Average Joe hold Grin? On exchange only. And we all know how bad decision is that. It's in first 3 advices you give someone ("don't keep your coins on exchange")
BTW, even terrible projects, some obvious scams and money-grabs have Windows GUI wallet.
So, the fact that Grin doesn't have it is intentionally for sure. Why? I'm not sure.