Usually because it's indicative of a lack of vocabulary that's supposed to diminish with age. It's broadly speaking a failure for an adult to not know the actual word for whatever, in their language. There's exceptions for when the language legitimately doesn't have a word for said whatever, but unsurprisingly enough those exceptions decrease over time. Other set when it's particularly obscure, too, but eh. Can also be considered as trying to disrupt or intentionally muddle communication with someone, which often held as immature at best (because you're intentionally miscommunicating, something that is rather disrespectful of the person you're talking to, a behavior that adults are supposed to have outgrown) or malicious at worst (i.e. attempting to confuse someone, something easily construable as hostile). Probably other stuff, too. There's usually a good handfuls of various reasons (of varying legitimacy) behind this kind of thing.
If it's in a more playful sense, well, that's mostly a continuation of the common adult affliction of being under the grotesque illusion of having outgrown illusion. For some mad reason it's a common cultural thing for adults to disdain fun things that younger individuals also find fun, so unstructured wordplay is seen as childish, or at the least directed towards a young audience. Exceptions there, too, particularly with poetry, but *shrugs*
Let it not be said that consistency is a virtue oft practiced by mankind.