The principal-agent relationship between a group of voters and their representative can sometimes be imperfect. More to the point, a representative could decide to go against the wishes of voters on a regular basis. Why is this an inherent risk of the relationship?

The interests of the agent and the principal may not be well aligned.

Principals can force agents to do things against their will so much that the agents rebel.

Agents and principals in this relationship can both be irrational at times.

Voters are often ignorant about the ins and outs of major issues, giving representatives the moral duty to decide for them.