Well, they're not really new groups - militia and the klan have been around for decades. They just like giving themselves new names because, like all angry idiots, they start infighting and splintering from each other.
Although, yes, the GOP have been enabling and emboldening them for the last (at least 5 but really many more) years - now they have a whole bunch of people who support them in the WH that are dismantling anti-WS task forces etc.
Most of the ones that participated in the march in Charlottesville were genuine new groups: Identity Evropa, Vanguard America, Traditionalist Worker Party etc.
And most of their membership is not a split from KKK etc. It's new people, often with very different backgrounds age- and education-wise. And one distinguishing thing about them is that they put a lot more emphasis on politics and outreach compared to KKK, and they openly reference fascist and Nazi ideas - which is to say, they don't just talk about white supremacy, but they talk about fascist ideals for governance etc.
Make no mistake, this is a new wave of political right-wing authoritarianism in US, not the same old.
That explains why they exist, but not why new groups form today still.