The idea of having staff goes back quite a long time, and you do need them. My first job was as an administrative aide to a state rep back when I was 20. I probably answered the phone 50x per day and drafted another 30 letters answering constituent mail each day, and when I wasn't doing that, I was helping constituents who were getting the runaround from state agencies. The more senior people researched and read legislation, and when I was off the clock, I stayed in the office and joined them to help and as a learning experience. I can't imagine the member doing all that himself and being on the floor to debate and vote. And a member of Congress has about 5 or 6 times as many constituents. Pretty much impossible.
But that's a separate issue. They're not getting rich off the pay. They're mostly getting rich off of side deals that are enabled by sloppy ethics rules.