Because the law says they have to.
Water usage in terms of total volume is meaningless and certainly an overblown concern on a global scale. In fact, it is not a concern at all on a global scale, but it can be a local problem. If a datacenter uses water normally intended for households and the infrastructure is not designed for this, it is definitely a problem. And that's a good reason for having to report water usage, not just to punish them, but also to plan distribution accordingly.