I fail to see how these things are one and the same. I get the point you are making, I just don't agree with it.

2+2 is a complete expression, the other is grammatically correct but logically flawed. Where is the logical fallacy in 2+2?

You're not making a fair comparison.

"What's 2 + 2" is a completely abstract question for mathematics that human beings are thoroughly trained mostly to associate with tests of mastery and intelligence.

The car wash question is not such a question. It is framed as a question regarding a goal oriented, practical behavior, and in this situation it would be bizarre for a person to ask you this (since a rational person having all the information in the prompt, knowing what cars are, which they own, and knowing what a car wash is, wouldn't ask anybody anything, they'd just drive their car to the car wash).

And as someone else noted there are in fact situations in which it actually can be reasonable to ask for more context on what you mean by "2 + 2". You're just pointing out that human beings use a variety of social mores when interpreting messages, which is precisely why the car wash question silly/a trick were a human being to ask you and not preceded the question with a statement like "we're going to take an examine to test your logical reasoning".

As with LLMs, interpretation is all about context. The people that find this question weird (reasonably) interpret it in a practical context, not in a "this is a logic puzzle context" because human beings wags cats far more often than they subject themselves to logic puzzles.