Its not 6 months, it's actually closer to a year.
They design the overall block, design cards, refine the cards, refine the limited environment, then hand the set off to the creative department. Creative sees the cards, comes up with art and creative things, which ends of affecting mechanics and creature types, which ends up affecting design. This goes back and forth for a while. Everything eventually gets finalized, then marketing starts slowly hyping the set for months. Marketing makes lots of promotional materials up during this time, so there can be big banners and posters and everything in local stores at least a month in advance. The masters get sent to the printers about 3 months before the sets come out, so they can start printing test sheets, make sure the foil overlays look right, etc.
You're right, they COULD shove cards into a set late in the process, but they never have before and they're not going to start doing it in the future. Throwing something as big as fetchlands late into the process would throw off all the testing they do for the standard environment, and that testing happens over a year out. Development frequently jokes about how they don't even know the names of the cards, because when they were playtesting with them, it was a year ago and they were called things like Angry Birdman and things like that. And they also often talk about how they have to ban cards strictly because of the set design lag time, they print cards to counter imbalances they saw a year out, (aka printing Rest In Peace in RTR to counter Snapcaster or Unburial Rites shenanigans from the previous block) but they cannot print cards to react to the live tournament environment.