Is there any reason given in Spirited Away or Japanese mythology that explains the frog-like spirits?
I don't think, that there is a reason given, but considering how much the bathhouse workers, which are spirits of various animals, despise humans, it is pretty obvious, why they don't want to look too much like humans.
Most male workers are frog spirits, most female workers are slugs. Lin is a weasel spirit and Kamaji is a spider spirit.
Since you were asking about Japanese mythology, the only folklore tale, which I could think of and find, which had men change into frogs and women change into slugs, is The Tale of the Gallant Jiraiya. It is a pretty popular tale, which is also used as a source for other manga/anime(i.e. Naruto). But I don't know, if Miyazaki really used this tale as an inspiration for the workers in the bathhouse.