Earth may have been one of the founding members of the Federation, but how much sense does it make for Earth to still be part of it? Earth thinks of itself as a Paradise. The Federation is only as much of a Paradise as its worst off worlds and its most dangerous, chaotic colonies. Earth is at peace. Looking at the Federation, it looks like it's been at war more often than it hasn't been. Kind of like America. Starfleet, the not-military branch of the Federation, has Humans in it that act more "primitive" than most Humans, if we look at what Gene Roddenberry describes in his TMP Novelization. Technically not canon, but it shows what the creator's intent was before everything except TOS and TAS was made. I think Earth outgrew what it created. They decided, "We can't be part of this anymore." And, from their point of view, they might not want to rejoin, even if Discovery is able to reconnect all the former worlds of the Federation.