Clients can't predict specs for the same reason developers can't predict budgets. Both parties are part of the invention process. They don't know what they want

Clients can't predict specs for the same reason developers can't predict budgets. Both parties are part of the invention process. They don't know what they want until they use it, you don't know how to build it until you try. And when these things are predictable, it's because they aren't novel. And since software is (ideally) non-repetitive, value fundamentally comes from novelty. In other words, the better you code, the less predictable you will be.