Why are raincoats made of synthetic …

Because synthetic fibers are more hydrophobic than natural fibers.

Cotton, linen, wool, hemp: these fibers are absorbent, and will take on water until they are replete.

Nylon and polyester, the two fabrics most commonly used in raincoats and umbrellas, are not naturally absorbent the way natural fibers are. They naturally resist waterlogging, and with a little bit of treatment they can be made quite waterproof.

Cotton and other natural fibers, on the other hand, require a lot of treatment to become suitable for wear in heavy rain. And everyone who has ever been around a wet wool coat or sweater knows that wet wool smells like a wet dog.

Because synthetic fibers are more hydrophobic than natural fibers.
Cotton, linen, wool, hemp: these fibers are absorbent, and will take on water until they are replete.
Nylon and polyester, the two fabrics most commonly used in raincoats and umbrellas, are not naturally absorbent the way natural fibers are. They naturally resist waterlogging, and with a little bit of treatment they can be made quite waterproof.
Cotton and other natural fibers, on the other hand, require a lot of treatment to become suitable for wear in heavy rain. And everyone who has ever been around a wet wool coat or sweater knows that wet wool smells like a wet dog.