This is an actual conversation between a friend and me yesterday afternoon.
Monty: I’m not going anywhere tomorrow, so I’m having curry for dinner.
Friend: Why aren’t you going anywhere tomorrow?
Monty: I’m not going anywhere tomorrow because I’m having curry for dinner!
Anyone know if this kind of joke has an actual name?
Misdirection?
The humor from many jokes comes from twisting an expected outcome around. Your example flips the cause and effect— in the first sentence, the cause is not ‘going anywhere’ and the effect is ‘having curry for dinner’. Then it’s revealed that having curry for dinner is actually the reason for staying home.
Well, if it weren’t a set-up for a joke, but were presented as serious reasoning, it would be “begging the question.”
That’s some catch, that Catch-22…
Are you sure it’s a joke 
Well, Heinlein 's assertion is humor is something bad happening to someone else, so for those who aren’t me, it’s funny. (Mind you, I say this from the loo. 
“Plop, plop…Oh, what a relief…”)
I think your user name answers my question!
Why do giraffes have such long necks?
Because their heads are so far away from their bodies.
I tell this to my high school students and inevitably one student laughs and the others think it’s s real explanation.
Another actual conversation:
Stopped for directions once… “How do we get to Point Fermen?”
Answer: “Point Fermen? Where is that?”
Try non-sequitur for size:
A non sequitur (English: /nɒn ˈsɛkwɪtər/ non SEK-wit-ər, Classical Latin: [noːn ˈsɛkᶣɪtʊr]; “[it] does not follow”) is a conversational literary device, often used for comedic purposes. It is something said that, because of its apparent lack of meaning relative to what preceded it,[1] seems absurd to the point of being humorous or confusing. This use of the term is distinct from the non sequitur in logic, where it is a fallacy.[2]
Notice it didn’t say it had to be actually funny.
Another actual conversation (which I have shared before):
Nephew: Are there porcupines in Michigan?
Me: I think so. I know they live near mountains.
Nephew: Are there any mountains in Michigan?
Me: Yeah, in the Upper Peninsula.
Nephew: What are the mountains called?
Me: Porcupine Mountains.
mmm
The humor comes from confusing cause and effect, or (as Lt Columbo once said) “putting the cart before the horse.”
I’m sorry, I can’t hear you; I’ve got a banana in my ear.
I’ll take it!
Another example, from The Goon Show:
In my high school’s variety show, it was a carrot.
I remember a non-observant Jew on this message board talking about how he was going to fast anyway on Yom Kippur, because how else would he have room for the big feast on the next day when you break the fast?
A conversation I had when I was teaching English in Czechoslovakia:
SHE: You know there are two kinds of cherries, red and black?
ME: Yes…
SHE: What are they called in English?
ME: Red and black cherries.
When I saw she obviously didn’t believe me, I added “Or sweet and sour.” (Czech, like Russian, has a different word for each variety.)
Another time:
SHE: You know the men who do manual labor at this school?
ME: Yes…
SHE: What do you call them in English?
ME: Engineers.
When I saw she really didn’t believe me, I added “They used to be called ‘janitors,’ but they don’t like it because they do more than just clean. They keep the physical plant running.” (Calling someone an “engineer” in Czech apparently implies they have some kind of specialized higher education.)
“Operator, what’s the number for 911?”
In English, too. Which is why nobody ever actually calls janitors “engineers”, even though their official title might be “sanitation engineers”.
And someone who does more than just cleaning, and also does maintenance, is usually called a “custodian”.