There are many terms for which Folk Etymologies are in wide circulation. Some of them are merely whimsical inventions. Others are very plausible and the only thing that prevents them from being given in the dictionary is the need for proper linguistic and historical evidence. Simply sounding plausible is not enough - the histories of words presented in dictionaries are based on the Principles of Etymology and are supported by linguistic and historical evidence.
Also limited space in dictionaries means that there is little room to explain why favourite stories are etymologically wrong.