Because my intention seems to be a unclear for some people.
Firstly you can still forfeit gammons and backgammons if you simply want to continue and end the game early.
And, as soon as you've taken your first pawn off the board, and gammon is impossible. You can still forfeit a single point to end early.
If you're not yet home, but it's clear the opponent will win. They can still double to end early. If the cube is not available to them. It will at worst, take you a handful of turns to get home and bear off 1 pawn to make the forfeit available.
In the cases where you'll likely take 5 or more turns before you can use the forfeit. That's a large enough difference that your opponent should never accept your single point forfeit anyways.
This change won't stop you from using the forfeit option. And it will never force you to play out long games for nothing. At worst it might have you waiting 2-3 turns extra, once every five games or so. That's not a big sacrifice to prevent abuse and promote fair play.