I imagine it’s just an ingrained use of language picked up from other people - no thought behind it, like an atheist who says ‘Christ’ as an expletive.
I was about 45 by the point I realized most people to varying degrees actually meant that they saw things when imagining them.
And so to me it's not that there no thought behind it, because I do something when I "imagine" something, just not the same thing. The word means something different to me, and most of the time you wouldn't notice even when it's staring you in the face.
E.g."I imagined her face" has a meaning to me even though I can't literally see a face when imagining it, but I can recall qualities and emotions and sensations of it anyway and to me those things add up to the meaning of that word, because nobody ever thought to assume it needed explaining that it was literal.
Opening that line of inquiry quickly makes it apparent that we assume a whole lot about how similar our inner lives are that just doesn't consistently hold up, but that we don't challenge without digging into precisely how people use various words.
Do people actually close their eyes and hallucinate or am I reading way to far into this?
In these exchanges the language typically gets sloppy and one person's "see" gets translated and reacted to as whoa, with their eyes?.
If you can imagine a red car and see it in your mind's eye (fairly ephemeral, especially if you force it, and nothing like as vivid as a hallucination) I think you're pretty normal from having followed a few of these threads over the years.
I know the distinction both from comparing with dreams, where I do see, and with a single experience during meditation (which I've tried many times but so far failed to replicate). They're nothing alike what it is to "imagine" for me.
It's not vivid like a dream though, and doesn't have that 1st-personness either.
Only 2-4% or so see nothing at all.