Earlier quoted context omitted.
Yay & Nay votes aren't the only job of legislators. They're often responsible for writing good legislation too -- something that's arguably difficult to do if you're new to the job, so the task (largely) falls back to special interests to "help" with the process. By way of example: Literature. It's easy to read a good book; it's much harder to write one without lots & lots of practice.
Even when the outside interests don't write the legislation, it's the staff and not actually politicians that too: politicians are basically constantly in meetings / at events and have no time for such work. :| Nothing prevents the staff from being retained from when legislator to the next.
British series "Yes, Minister" / "Yes, Prime Minister" immediately come to mind.