This is an early paper on the efficiency of different fabrics for virus filtration. (Lots of testing as gone on since then.) However, it nicely illustrates breathability, the other factor to consider when making a mask:
They summarize nicely here:
Although the dish towel and the vacuum bag captured the most particles, they were also the hardest to breath through. With two layers, the dish towel was over twice as hard to breathe through as the surgical mask.
If you can't breathe trough it, you won't wear it for any significant length of time, making it useless as a mask.
I don't agree with the conclusions; as I said, a lot of research has been done since then. But it has a nice illustration of breathability, and why it matters.
Why is recommending cotton face masks instead?
Although this paper doesn't answer the question specifically (as they jumped from cotton jersey to tightly woven pillowcases, the answer is it has decent filtration plus decent breathability, i.e. it is somewhat effective, and it will be worn.