Lack of trust between the rent-seekers and the politicians, due to the inherently underhanded nature of the deal and the unavailability of both legal recourse and reputational incentives to enforce compliance, pushes down the price that politicians can demand for favors.
Regulatory capture is a related concept which refers to collusion between firms and the government agencies assigned to regulate them, which is seen as enabling extensive rent-seeking behavior, especially when the government agency must rely on the firms for knowledge about the market.