Why can humour be dry but not wet?

In this case, dry is not the opposite of wet, but it means bare, and lacking adornment, such as a dry report.

Etymonline, the online etymology dictionary, indicates the word has been used to describe comedy for more than 500 years!

dry O.E. dryge, from P.Gmc. *draugiz, from PIE *dreug-. Meaning "barren" is mid-14c. Of humor or jests, early 15c. (implied in dryly); as "uninteresting, tedious" from 1620s.

As for antonyms, instead of a dry lecture, you might sit through a lively one. When applied to jokes, a joke might be farcical, or whimsical; a comedy routine might be laced with slapstick. One website I found listed 20 distinct forms of humor; others in that list which seem "opposite" of dry include hyperbolic, sophomoric, screwball, but probably not mordant.

While it is true that dry is often used to describe a type of understated humour, and the person who tells a joke usually delivers the punchline in a flat deadpan voice. It is false that its most obvious antonym wet is never associated with humour. You can in fact wet yourself laughing, which implies that a person laughs so heartily that he or she temporarily loses control of their bladder. Another typical slang expression is piss yourself laughing

A plausible antonym to dry humour could be vaudeville, as in

His joke definitely wasn't dry, it was pure vaudeville!

Although I still prefer J.R's slapstick solution, which typifies the physical humour/comedy genre.

While I have not seen such a usage, the term juicy might convey the sense of the opposite in both these examples.

In regard to humor, the term dry can convey subtlety and indicate a form that may be an acquired taste, like dry wine or martinis. Juicy connotes a more robust and obvious quality.

As to the lecture, the concept of bare or barren does seem to be the closer meaning. I think juicy would work there as well.

This is mere opinion and without reference.