QAnon seems to be used for the narrative (dangerous nazi racists etc)
They can't use "Q" because that is too specific, they would have to attack one person "and his followers" and that would raise too many questions in the reader "who is Q?" and that might lead the reader to wonder why the reporter doesn't just ask who Q is, or if POTUS or IAs know who Q is.
There's a subtle difference, and you'll note POTUS was asked about QAnon, the movement (and the anons), not about Q.
He can quite plausibly say he's "heard about" QAnon because as you point out, we know Q as Q. In general, while I'm aware of the term QAnon, I've never really felt part of that "movement". It's a kind of secondary (and good) effect of the GA, but I don't consider it to be what we do here.
Maybe that's unfair and QAnon has fed back in here now, but that's just the way I've always seen it.
Anyway, it's easier for the media to attack an amorphous group of people, clearly visible at rallies etc., than to tackle the actual issue of Q's existence and what Q is asking (and heaven forbid it's a group and not a 'he', kek)