Why do people never say the following:
I have wanted to ask. . . .
Maybe it would be better to ask whether it’s correct to say:
I have wanted to ask. . . .
But really I think I’ve never heard that form. Usually I hear people say (for example):
I wanted to ask how you spent this morning.
I’m talking about the situation when, for instance, one person is discussing something with other people. Suppose in the middle of the discussion, the person realises that they need to get some complementary information from the speaker. They wait for their turn to have a word, and when their turn comes, they say:
I have wanted to ask you, how do you make pastries?
That is, they wish to ask the question which occurred in the mind of the person, and they retained the thought until the moment they asked the speaker. Is it formally correct for them to use present perfect there?
Of course, there is another situation when you say:
Yesterday I wanted to ask you about your health, but you had fled. How are you now?
And here it seems quite okay to use the simple past.
Best Answer
I think OP's basic premise is mistaken (Here are about 5,700 results from Google Books for "I have wanted to ask", so it's certainly not true that we never say it.
And in some contexts ("I've always wanted to ask", or "I have wanted to ask for some time now"), Present Perfect is arguably more natural than Simple Past "I wanted to ask".
As OP suspects (and as backed up by his comment re 2,120,000 GB hits for "I wanted to ask"), the Simple Past form superficially seems almost 40 times more common.
Whilst I agree Present Perfect really is less common than Simple Past in OP's "polite question" contexts, the preference is nowhere near as marked as those figures suggest, since most of the 2,120,000 instances reference past time in a "narrative" context (where only Simple Past works).
But even allowing for that, I'm sure there's still a preference. I don't disagree with Barrie's point about Past Perfect Continuous (I have been wanting to ask) often displacing Present Perfect (I have wanted to ask), but there's at least one other factor in play here...
Both Past Perfect Continuous and Present Perfect imply strong links to the present moment. But in OP's primary context, "I wanted to ask [you] [some question] is often just a fairly meaningless "deferential introduction to an interruption" (a bit like the polite throat-clearing "Ahem...").
In such situations, the speaker is deliberately trying to create "distance" between himself/his words and the "present moment" (that's why we say "I wanted to ask" rather than "I want to ask" in the first place!). Obviously it would be counter-productive to use a verb form that's specifically adapted to linking events in the past to the present moment.