Because my priority is making the players feel like they are there in a world, not like they are playing a rules system. So if I want that, one way to do it is with rulings that can be one offs. You can codify them if you want, but if you do, you end up with effectively a heavily codified and rules heavy approach (which is often not what people are looking for). The way we do it is when something comes up, the GM figures out how to handle it then, if it isn't part of the core rules. But we don't write that solution down, and we almost never make use of it identically again (some patterns will emerge but they are more broad patterns than specific rules to remember). We also don't worry about whether this mechanical solution to the problem is the same as the last one. Our concern is, does this mechanical solution feel appropriate now
Obviously this isn't for everyone. But I have been doing it for ages in a variety of systems, particularly when using more rules light systems, and I can definitely say it can work perfectly well.