Towels can become hard after washing due to a number of factors such as fabric softeners and residue from dryer sheets building up on the fibers of the towels.
Or you could have hard water that is causing the towels to be hard after washing them.
If you don't have hard water then you might need to make sure you don't have any leftover fabric softener or laundry detergent residue in the washing machine.
Also make sure you're not adding fabric softener along with the laundry detergent during the wash cycle as fabric softener always goes in the washing machine during the rinse cycle and not the wash cycle.
You could also soak the towels in a bucket of fabric softener along with warm water for a few hours before you run the towels through a wash cycle in the washing machine as that sometimes helps to soften the towels and prevent the towels from becoming hard during washing.