It's probably best not to question the logic. Cartoons are known for inaccuracies. My only guess is because of the whole "anti-aging" in cartoons. It may have been a long time, but the gang doesn't look much older at all due to this rule.
An example of what I mean is Rugrats. They've had more than one celebration of Christmas, for instance, as well as Halloween, but they don't age. They're still 1 years old, 2, 3, etc.
So this can create confusion if you want to try to put chronological logic in something. Of course this rule does not always apply. Some cartoons, like Avatar The Last Airbender, do try a "real world time" sort of thing, making the events stay within approximately a year rather than many years (all the while making them look the same age).
