Except there's tons of internal logic the show tries to follow, Seth MacFarlane and other producers have admitted in interviews they try to not have most adult characters close to the main cast understand Stewie. Peter is stupid, everybody hates Meg, Carter is a greedy rich *******, Quagmire is a pervert, Joe hates his life, etc these are all consistent parts of the show that are rarely strayed from. This isn't Looney Tunes where in 1 short Daffy's a sci-fi hero in the 24th and a half century and the next he's a Dick Tracy parody.
You can't talk about serious issues as much as Family Guy does then hide behind it's just a cartoon when people think deeply about it. Besides the show has established internal logic that everyone understands Brian. So when he's talking to Stewie in a way you'd talk to a much older person you'd expect someone to ask him why he's trying to have an intellectual conversation with a baby.
Asking a question doesn't mean I think this is ultra serious business if you don’t like this thread topic just ignore it, I don't need your condescending "calm down it's just a cartoon" responses. Move on if you think this is a stupid thread.