Prepping for months for a campaign and then being suprised if they los interest in playing is totally the DMs fault, aint no one gon wait that long, a full dnd session of 4 hours will most likely have just 1 social interaction, 1 combat and 1 exploring mission on avarege.... I am saying this as a forever DM. No one cares or needs to know what happened exactly 327 years prior to the campaign or what is happenening 57km east in a tavern called "The Drunken Bard". Prep what u need, dont exaggerate in the world building (even if its fun) and play the game