That's a good question, and the reasoning you brought up definitely applies in a lot of cases...but I think one that you overlooked is that, for everyone to come in and tell you they like your work at the last possible second just feels...insincere.
Like, you had all these chances to say something. There were probably instances where I asked for your opinions and feedback, or tried to do things based on what you wanted; you, the audience that I genuinely wanted to entertain at one point or another. In the context of a story, there were probably big important events and shocking twists and revelations that I thought you might have some reaction to.
And I got...crickets. For all I knew, you just didn't care.
But NOW that all of a sudden your art-machine is leaving, the content stream is about to be cut off, NOW you care. None of my decisions were worth your commentary EXCEPT my decision to stop entertaining you.
The situation itself is almost dehumanizing, and after all the hurt you felt working on your art and feeling like no one actually wanted it, for many it could be the last straw. Like, it'd probably be healthier to put the whole thing behind you and move on, whether these people mean what they're saying or not.
To give a real-life example: I still remember that thread I made where I said I wasn't going to post novels on Tapas anymore. I was all depressed and I vented all my frustrations...and as if by magic, people started searching out my most recent novel and giving it likes and subs. As in, I never gave them any links: they went to Tapas and looked it up of their own free will, just to support me in my hour of sadness. Not even previously-silent fans, people I didn't even know, who had probably never read any of my work before!
And at the time I just...laughed (and deleted the novel anyway). ^^ If I were younger, I would have thought this was a sign I shouldn't quit; that maybe I could use these pity subs to build a 'real' audience. But I'm old, I've been through it before, and I know it wouldn't have been worth it.
I'd always know why they were there. And they'd probably always know why they came: because I was sad and they wanted me to not be sad. That's not the same as liking someone's work...no one wants to follow someone out of guilt or obligation; even if you genuinely want to like what they're doing, there's always this pressure that you HAVE to, or else you'll be letting them down.
Is that what you want, as an artist? An audience who stays only because they feel it would be rude to leave...? I think it's just as bad, if not worse, than having no audience...which you'll eventually end up with, as these people mature and realize they don't owe someone their continued attention just because they were sad one time. It's just a bad deal for everyone involved.
I get that: sometimes it's hard to construct a comment that says what you really want it to say. But if you think it's important, you can always...work at it. A little? Plan out what you want to say, look at other nice comments that you think were written well? Something...?
Like, if it's worth doing, it's worth the effort....and in all honesty, this shouldn't take much effort. Writing a comment isn't like writing a book; it doesn't have to be amazing to make the recipient feel good about reading it. It can be something as simple as telling them one thing you liked about what they did, or how much you've been looking forward to it.
If anyone wants advice, I could start a thread about writing comments. ^^; I'm not trying to be mean, but...it ain't rocket science, dudes. With a few pointers and some practice, there's no reason you should be incapable of saying something nice to an artist when you want to.