The problem is getting people to do the grunt , non prestigious, repetitive, (relatively) low paying overseer job without the hope of making it to the trading floor. Which is why the separation isn't that tight.

Also, there aren't that many people who know all that well the workings of the financial markets, and the computer tools user by the SG (which are mostly done in-house). Taking somebody from the middle or back offices, or support, is cheaper and faster.

Un roi sans divertissement est un homme plein de misères