This process does not happen out in the open for everyone to witness. It occurs within a committee system that was designed to help interest groups get what they want. It also has the effect of stabilizing the government's spending patterns over time. The congressional committee structure has evolved into a system through which informal agreements between members are upheld and enforced over time. Committees are repositories of policy expertise.
Committees are policy incubators.
Committees possess disproportionate control over the agenda in their policy domains. Committees are deferred to, and that deference is reciprocated. This means that, instead of sorting out the facts for each issue or bill that comes up for a vote, members of Congress are willing to defer to a committee and grant its members leeway to make important decisions regarding legislation in their areas of expertise. Committees can do things to discourage opposition to their actions. They can induce cooperation not only by threatening to oppose dissenters' legislation after a bill is voted down, but also by threatening to attach unpopular riders to a bill before it reaches the floor.