Brake fluid is hydraulic fluid.
By using the same fluid as the brakes, there is one less type of fluid to be stocked and installed by the factory.
There is no chance of using the wrong type fluid in the wrong reservoir by either the factory or the owner.
There are components in the system that are shared between the brake and clutch, if different fluids were used the specs of the shared parts may need to be different thus increasing the number of stocked items and the possibility if installing a wrong component in the wrong system.
Water absorption is not an asset, it is a bad thing, but not bad enough to use something else.
Bottom line is that it is cheaper to use the same fluid and it does get the job done.
Stocking units are a big deal to a large company.
Besides, if you spill some of most hydraulic fluids on the plastic or paint it just makes a mess that has to be wiped up, if you get messy and spill brake fluid on your plastic or paint, they get to sell you new parts.