In general, it is because the fibers are not hydrophobic, and the spacing between fibers is too large to keep water out.
not waterproof
Proof against penetration or permeation by water; impervious to water; as, a waterproof garment; a waterproof roof., A substance or preparation for rendering cloth, leather, etc., impervious to water., Cloth made waterproof, or any article made of such cloth, or of other waterproof material, as rubber; esp., an outer garment made of such material., To render impervious to water, as cloth, leather, etc.
Gore-tex
cause they are waterproof
To effectively make cloth diapers waterproof, you can use a waterproof diaper cover or apply a waterproofing treatment to the fabric. Waterproof covers can be made from materials like PUL (polyurethane laminate) or wool. Waterproofing treatments like lanolin or silicone-based sprays can also be applied to the fabric to repel moisture.
Waterproof cloth diaper covers for babies offer benefits such as preventing leaks, reducing diaper rash, being reusable and eco-friendly, and saving money in the long run.
Waterproof cloth diapers for babies offer benefits such as reducing waste, saving money in the long run, being more comfortable for the baby, and potentially reducing the risk of diaper rash.
Umbrellas are made from a synthetic cloth called sail cloth. Several specialty fabrics are made that have a coating on them to make them waterproof.
In the 1940s, waterproof diapers typically consisted of cloth diapering systems that were covered with a waterproof outer layer, often made from rubber or plastic. Parents would use absorbent cloth inserts to soak up moisture, while the waterproof cover prevented leaks. These diapers required regular washing and drying, and were often fastened with safety pins. The introduction of disposable diapers in the later decades would eventually change diapering practices significantly.
Gore-tex
There's really no telling. As soon as people were able to weave cloth enough for it to be turned into everyday clothes, people have been looking at ways of making it waterproof. And before that, people tried to figure out how to make leather waterproof as well, usually by greasing them heavily.
On the sailing ships, a pall was a heavy canvas cloth covering, sometimes coated with tar to make it waterproof.