They probably enjoy working at Jagex, enjoy their pay package each week / month enough to not feel the need to do so, that said it's also likely stated somewhere in their contract that there would be legal proceedings by Jagex if they were to do anything like that and loss of job, bad references etc, would be the same if you stole from any company you worked for... Wouldn't be in their best interests - My beliefs
.. Saying that there is always the odd one or two who do 'side jobs' in every business , trade etc. So who knows.