It's been going on for years because Egypt sucks at negotiation. They can easily bomb the dam, it's not like they have a primitive military like the rest of Africa.The Nile negotiations will be going on for many years I reckon.
Egypt curled up in a ball and let Ethiopia build the giant dam. If you think Ethiopia won't use the dammed water for agriculture you are naive.
And you must be naive if you think the Ethio highlands are suitable for industrial agriculture. You do realize part of the reason both Egypt and Ethiopia are building railways to Sudan is because they know it's the only country out of the three that is fit for mass food production? That's why Ethiopians are so focused on electricity and other manufacturing (such as textile). They know it's easier and cheaper to just import from a neighboring country that has the arable land.