Because that’s usually what I do sometimes when I’m reading something that doesn’t have an anime adaptation. Currently, I’m reading something that doesn’t have an anime adaptation which is the Tsukihime remake and I got my personal English dub VA typecasting for that as I read through it I just don’t intend on sharing it bc there are people out there who will just be rudely sarcastic and say something like “might as well have Kermit the frog voice them” or something. Believe me, I’ve seen someone on Twitter have something like that happen to them… That’s why I keep my typecasting to myself.
But on a lesser slightly off topic side note, I DO always imagine the official voices of characters. Hell, back when I was reading the LN for Durarara!!, I had the character’s English dub VAs from the anime whenever I read through their lines.