Because I did this year, for the first time ever.
Here's what's weird about this, though: I got the idea from a comic strip. I read that line about scrubbing "the gunk out of the tree stand" and I thought to myself, "Do people scrub the gunk out of the tree stand? I've never done that."
So as part of my de-Christmas-ing the house this year, I ran some soapy water through the Christmas tree stand. (Truth be told, I didn't really scrub the gunk.)
I don't know how I feel about it though. True, the tree stand looks as nice as it has since I bought it. And true, it does get awfully gunky. But it has no other use than to get gunky. It's not like from January through November we drink from the thing and then in December decide to prop up a tree in it. It has one use: get gunky from propping up a tree. (Unless 'sit in the garage for 11 months' is a use.)
At the very least, I've washed the tree stand for the first time in six years. Maybe I'll do it again in 2019.