Because creating a whole new model is easier than editing an existing wc3 model and hoping it would perform flawlessly.
But then, creating a model itself is not easy, especially when you're the kind of guy that set a very high bar on quality.
As you set the bar higher each time, you start to wonder why would you ever want to help others for free since it takes quite amount of effort.
Thus, why most of the modelers would only do freelance or paid works.
At least, that's my thought.