Because the water is locked up in zillions of cells, which are surrounded by cellular membranes, giving them a stiffness. A Tomato is 95% water. But it feels solid when you pick it up. Same reason

Think of it this way: Take a basketball. Cut a hole in it. Fill it with water. The basketball is now something like 90% water. But when you touch it, it feels solid. Shrink that basketball down to celluar size, and put a zillion of them together to make a human, and you have a solid feeling human that is mostly water.