If the game starts out by restricting resources, then turning off those restrictions makes the game easier by a set amount and is very simple. You go from some restrictions -> no restrictions. No more decisions necessary.

If the game starts out with no restrictions, then you can't simply "turn on" restrictions (unless the developer specifically provides optional restrictions). You're attempting to make the game harder (or more balanced) in this scenario, but you cannot accurately determine what level of restrictions corresponds to what level of difficulty, because you haven't played the game yet. At every step along the game you'll have to - mostly arbitrarily - decide when is a "balanced" time to restrict yourself, which again is near impossible to know without having already played through the game. And this turns into playing against yourself, when you should [I want to] be playing against the game.