Actually, there's an interesting response to that point, so let me tell you why I'm, as you put it, unwilling or unable:
(formatting it as a more general post as opposed to a direct response to your remark and inserting a bit of a straw man in the process).
One, it'll turn around on me.
Any date I mention now, no matter how much I emphasise that it's tentative, unreliable and might slip, will be a rock-solid date three quotes down, and when it does eventually slip, I'm gonna get roasted over it anyways.
Two, settling for less.
Any date mentioned now means feature lock. I might have a great idea for an extra scene, or an extension of an existing one, or expanding on the ending (in fact, I have a few ideas brewing on that), but I'll have to scrap all that and ship just what I can manage in the time remaining, because now there's a date and a roasting waiting for me, see point 1.
Three, what's the point, really?
We're not actually making business software, are we? There's no need to roll out so that major customer can go live before the next fiscal quarter, is there? No penalty clauses for costing them money. Neither is marketing breathing down our necks to hit that slot between two major competitor releases, or to have boxes on the shelves well in time for the holiday season.
In fact I'm not even a business, just a person making a game because I love to make a game, so what's the point of imposing a deadline if it'll only serve to reduce the quality and cause me grief (see 1+2).
But deadlines are great incentives to focus the mind and push that extra bit, aren't they?
Well, yeah, but really, the need to show some progress on the weekly progress update serves that purpose just fine without setting myself up for a reaming or endangering the quality of the end product, as a release date would.
But there's a demand for a date!
Is there now? I'm sure everyone serious about games of any sort is trained by marketing machines to hype and rave and rage over release dates, because that's how the business is run and that's what you do.
I'm also quite sure most of my supporters understand what I'm doing and agree as to how I'm doing it, or would if they read this, and would really prefer *not* to have a release date for the exact reasons I outlined above.
(note, this might be an interesting post to refer any future "when update?" question to).