Because both having a pen put upon them, create escapist thoughts.
A raven is an extremely intelligent animal, able to think outside the box.
A writting desk is open to a world created by thought.
A pen of either meaning, can instill thoughts of escape.
Like the references of other thought altering tools, in “Alice in wonderland”,
a pen can alter the mind of a free soul trapped inside.
Considering the typo, I think the more interesting question would be “Why is a raven like a Wittig reaction?” but I guess the comparison would only be valid if Caroll (or Poe) was hopped up on some phosphorous-containing compound.