Well, Ted, because I can't.
Now, spinning a yarn with a straight face, you bet. You would be shocked at what I can pull off... well, that's another thread...

But the prospect of practicing, learning, memorizing, and maintaining several "smaller-but-effective" pieces is so very much more attractive to me (and easier) than is (gasp) improvising something! My improv days are over, I'm afraid. I do stay on top of it enough to teach early improv skills to students, because it is SO important to me that my students get at least a start at this, and feel comfortable and welcome to create their own, no matter what level (and for theory & skills class it makes harmonization so much easier).
Wish I could do what you can do, but not enough to apply myself, I'm afraid.

Maybe I will recover from my laziness...