Selling time is the default in software services because it feels safe. The client thinks they're protected ("I only pay for the hours you actually work"); the studio thinks it's protected ("I bill for every hour I actually work"). Both are wrong, and the relationship is structurally adversarial from the first invoice.
When we charge for time, every efficiency gain on our side becomes a discount to the client. A senior engineer who solves in two hours what a junior would solve in twenty actively destroys margin. The economic incentive is to be slow, or to staff projects with the cheapest people we can find, which is exactly the agency model nobody wants to be on the receiving end of.
Project pricing flips it. We charge for the outcome, the client pays for the outcome, and our incentive is to ship it as well and as quickly as we can. If we're slow, we eat the cost. If we're fast, we keep the margin and reinvest it in better tooling, better people, and a maintenance retainer that pays both sides for years.
The honest version of this conversation, on the first sales call: "What's your budget?" If we can't fit a real scope into it, we'll tell you. If we can, we'll write it down, sign it, and not bill you again until the milestone hits. Hourly billing is a substitute for trust. We'd rather just earn the trust.