Legislator is a person that can write laws, it does not mean it's their duty to write laws. In the United States their duty is to uphold the constitution and represent their constituents. Not to create more rules.

I don't think the point is they have to spend their time writing more rules if they don't feel they have to, but they are the ones who would do it: with respect to the rules and policies and procedures of our government, we have a legislative branch that is supposed to draft and maintain them, an executive branch that is supposed to implement and enforce them, and a judicial branch that is supposed to interpret and adjudicate them... only, the boundary between the first and second (and arguably also the second and third) of those branches is a bit blurred as congress keeps writing vague laws that say "there shall be an agency which will prevent X" where X is something extremely high-level like "unfair business practices" and then the executive branch is forced to figure out what that means, and in the process is effectively doing the wrong jobs :/.

Unless their constituents think the current laws of the land are 100% perfect, then representing their constituents would involve passing laws.

And they would need to convince enough other representatives and the constituents those representatives represent, which they can't because the country is extremely divided.