Simply put: because I can knit something that looks like this:
And transform them into these:
These are my test pair of felted clogs. I could probably count on one hand the number of times I've used someone else's patterns, but sometimes, another knitter's patterns are so beautifully designed, so masterfully written, that I'd be foolish to attempt to reinvent the wheel. This Fiber Trends pattern is one such masterpiece. It's simple to knit, but requires the knitter's full attention to track the rows. The felting process is a leap of faith; you may knit for Women's Size Medium, exactly as planned, but there's no guarantee that you'll get there. This pair required four trips through the washing machine with various additives, etc. before it felted close to the size I require. Test #2 is on the needles.
That's another thing about knitting: even though I used someone else's pattern and the recommended commercial wool yarn, I'm able to customize the blank slippers to produce something unique. (In this case, I embroidered random stitches using my handspun yarns.) There is exactly one pair of these slippers in the world. All it took to get there was yarn, needles, basic knit and purl, hot water and soap and a bit of improvisation. What more could I ask?