Imagine an upright design that relies heavily on precise spacing. As in, spaced really tight. A black design, a condensed design, doesn't matter. Now how do I go about designing an adequate italic companion? Obviously I will have plenty negative sidebearings which will one way or another collide when set immediately next to an upright glyph.
In practice, italic is set in isolation, so the most of an issue is collision with punctuation. Italic and upright in a single word happen mostly in logotypes and are kerned manually.
But I can imagine a discrete italic axis in a variable font so that the kerning can be included within the font. Has that been done?