To the contrary, language games are quite plentiful. Regardless of the style, children learn these very quickly and can speak them amazingly fluently. Secret languages, however, though primarily used by children, have by no means been limited to them. Adults too have benefited from secret codes. For many, the benefit may be minor, speaking to your spouse in a way that your children can't understand.
Cockney rhyming slang is one such adult language game which has existed over a century.
Whatever its birth, it remains in use today, and is remarkably clever. For other adult languages though, the secrecy of the language may be a very important matter. For instance, back slang has been used by soldiers at war. Further, American slaves had many secret languages to help them keep everything from escape plans to covert education hidden from their masters. Slaves also developed Tut Language in order to secretly teach other slaves to read.