Most permanent recruitment is contingency: the agency is paid only on placement, competing with other agencies for the same role. The rational strategy under that model is volume and speed — submit early, submit often, and invest as little unpaid assessment time as possible, because most of it will not convert.
You are buying something different. A senior hire who is wrong costs months of salary, the opportunity cost of the role sitting unfilled again, the disruption to the team, and the credibility of the hiring manager who championed them. The interests are misaligned at the structural level, and no amount of relationship management fixes that.