Because making lazy, cliched jokes, that place transwomen as the butt of said joke, is still funny in 2018 - and, well, who cares if it serves to normalize transphobic attitudes in society which often lead to serious real life consequences for said transwomen, such as assault and murders?
And before some ignorant person pipes up and asks: Yes, the word "tranny" is a slur, and no, it is not the only reason this whole thing is grossly transphobic.
https://attitude.co.uk/article/new-amazon-show-jack-ryan-slammed-for-using-transphobic-slur-1/18963/