Simple: because that was a pretty lucky accident that probably is too risky to attempt to replicate.
Let me elaborate:
There are two comics that you should consider:
The first one is a retelling that explains HOW Obelix felt in the cauldron when he was about 6.
The story is narrated by Asterix, apparently to the conventional readership, and tells that in childhood, Obelix was often bullied by other boys, until Asterix, to assist his courage, induced him to drink some of the magic potion that made the villagers invincible. When they are interrupted in the act, Obelix falls into the cauldron containing the potion, and drinks it all, and is thereafter permanently under its influence.
As you can see, Obelix actually drinks the magic potion, not just "bathe" in it (so, sorry, no Achilles reference).
Now consider what happens in the second comic, Asterix and Obelix All at Sea.
In the story, Obelix actually manages to drink an entire cauldron full with the aforementioned potion. The results? First he gets turned into a stone statue, and then in a kid version of himself.
You may say that that effect was due to Obelix already having super-human strength when he drank the second cauldron... but the same comic show the same destiny happening to Crustacius, a roman Admiral- who never fell in a cauldron when young!
So, apparently, drinking too much magical potion in a single go may have dire consequences. Based on the results, you may even say that it will have dire consequences most of the times! This is somehow implied in multiple occasions during the series: every single time Obelix is given some magical potion (for example during the pyramid escape in Asterix & Cleopatra) Panoramix (or Getafix if you prefer his English name) always try to give him as little potion as possible (hinting he may already have feared that overdosing it may cause problems).
It is also worth noticing that even when Obelix originally fell in the magic potion that wasn't without side effects either: before the incident, Obelix is shown as being extremely pacifist. While that personality trait may just originate from fear and the inability to defend himself, it is undoubted that he changes after gaining his characterizing strength. Even his usual over-the-top reactions to being called "fat" - "WHO ARE YOU CALLING FAT" - seems to be something he developed after the incident.
At least I think we can assume Obelix appetite isn't a potion side-effect: based on the first comic, "How Obelix Fell into the Magic Potion When He Was a Little Boy", he was already somehow overweight before the incident