1. The word is not found in the usual dictionaries, so the D&D youths who first encountered the word just assumed it was pronounced the way it was spelled.
2. An evil legendary sorcerer's name sounds like it SHOULD rhyme with "witch" - another evil legendary sorcerer.
3. It's pronounced "like?" Wow, nothing likable about those guys. The word as pronounced "litch" sounds like "itch," "witch," "bitch," and other unsavory terms.
So it stands to reason that only those with an Irish language background may assume the correct pronounciation. Me? I'll stick with the wrong way of saying it.
Besides, they're my favorite undead critter and I'm not about to call them something different.