America doesn’t work like that. Due to the way the electoral system is set up only two parties can be viable at any given time. The only time you can create a new party is in a realignment, eg. When Lincoln formed the republican party as an explicitly anti slavery northern party and the democrats became the pro slavery southern party. Prior to that it was the whigs and the democrats, which were aligned around issues other than slavery.

So unless the progressives can force a realignment along some new issue and subsume a major party, creating another party would just be handing a lot of elections over to the Republicans.

This is why lemmy was so vehemently against Jill stein and the greens even though most people on here agree with her politics more than kamala, because they saw her as sabotaging the democrats chances and handing the election to trump.

Also hijacking a party is easier than building a new one and if you gain control you can use the power of that party to shift opinion. Most voters these days vote based on there team, or more recently against the other team. So if the democratic party started pushing a green new deal then the rank and file would fall in line, if nothing just to stop the Republicans. You can see this in how trump hijacked the Republicans. If you showed a republican project 2025 in 2015 they wouldve called it absurd, but now most are in support of it.