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.
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.