It's because most of the time, people asking questions like these don't actually have a perf issue. This is very evident on StackOverflow.
So when you do have such an issue, and you want to make it clear that it's not a premature optimization, you should mention that it's a hot path as determined by profiling (or whatever other technique). Then you won't get the Knuth quote.
On the other hand, it’s highly annoying to have to anticipate and ward off a whole range of uncharitable and/or unimaginative assumptions about why one would want to do such a thing before there is even an attempt made at a straight answer. If nobody knows the answer, I’d rather have no replies than yet another soliloquy on why nobody actually needs to optimize anything.
It's a function of what's typical. You might be coming to it from the position of having just this one issue, but keep in mind that the people answering that are likely seeing more than one such question every day, and most of them are premature optimization. Thus people bring this up, because in the grand scheme of things, it saves a lot of time for everybody involved.
The person asking may be prematurely optimizing, but can you make that assertion for every single person arriving at the same question/post from a search engine? If premature optimization is a bad thing, then prematurely calling out premature optimization is doubly-so.
I have no doubt that fielding ignorant questions every day inculcates a certain attitude of presumptuous arrogance. How could it not?
However, as the sibling comment says, I'm most commonly annoyed by this when I arrive at an answer via search and have to wade through the whole interrogation process about what the OP was really trying to accomplish.
It's like, look... you're not being brought on as a consultant, where whole-system analysis and root-causing issues would be part of your professional responsibilities. Sometimes a person just wants to ask an obscure question about software to see if maybe someone else knows the answer. That person shouldn't have to provide a complete backstory and concept of operations for whatever it is they happen to be working on.
They're also not entitled to an answer, so if you don't know, for the love of god, just move along and do something else with your day. I think this kind of behavior wastes a lot of time for everybody involved.
So when you do have such an issue, and you want to make it clear that it's not a premature optimization, you should mention that it's a hot path as determined by profiling (or whatever other technique). Then you won't get the Knuth quote.