Perhaps I mean to ask a question then, how did FDR manage to become such a widely heard leader back then with so many less ways for people to talk together? Did it make a bigger difference that he had to exist as someone people spoke to other people about? Shouldn't it be easier to find these leaders with so much more access to everyone nowadays?

Communication friction is only one cost of running a campaign among many, so the structure of parties and campaigns and primary / general elections has largely remained the same. Even if the technological barriers went away, I suspect the human factors would still hold up the structure because only so many people are willing to spend years of their life building legitimacy and promoting a political platform and each voter is only willing to spend a certain amount of time participating and choosing.