Because they are language professionals, who enjoy studying languages and packing as much power as they can into a language -- computer or human.

In my experience, linguists are not terribly punny, though -- what accounts for this difference?

(FWIW, my first-hand experience has mainly been with psycholinguists and functional programming folks)

Linguists are language analysts whereas programming language people are language creators; perhaps this explains why the later create more wordplay?

I am not sure if this applies, but in my experience, presenting a dry, more or less theoretical subject matter in a humorous way makes it substantially easier for me to stay focused.

Also, as somebody else already said, if you are designing languages professionally, you probably get to appreciate puns on a whole new level.