Because I don't have enough yarn. Or things to do. Or knowledge.
Sometimes I see or hear comments about not using hand spun for weaving. Or, you can use it, but only plyed. Which is completely contradictory to the historical record.
I have used both commercially and hand spun yarns for warp and weft. Some yarns are more tender than others and may require sizing to add strength. But yes, you can weave with hand spun singles.
After extensive consultation with a master spinner and multiple samples, I now have the yarn to make 'something' - in this case three scarves - one for the dyer, one for the spinner, one for me.
The spinner and I drew on our respective personal database of knowledge, consulted extensively, then she set about making samples of the hand spun singles which I then wove. Because that is how you find out if your extrapolations are correct - you sample. Change one thing and everything can change.
People sometimes ask me if I sample any more. The answer is, as so often, it depends. I now have an extensive pool of knowledge to draw upon, and frequently I will not make a sample when I am working with known yarns using a known weave structure.
But when I get a new-to-me yarn, yes, I sample. That is how I find out the limitations of the yarn, and the hidden beauty that may be revealed in the process...including wet finishing.
Working with other fibre workers with complementary skills means that we all benefit. I don't have the technical skills to make such a consistent yarn (in thickness, in twists per inch), and she doesn't (yet) have my weaving skills. The dyer contributed by making yarn colours to our taste and that means that all three scarves are different colours, not all natural white. By working together we learn a lot more, a lot faster, than if we had each tried to do this exploration from beginning to end.
Curiosity. Love it!