Why use a Realtor that is also an investor?

So I have heard that when using a realtor that it is a good option to use a Realtor that is also an Investor. I understand the reason being would be because they are very familiar with investing obviously. My question is, what stops a realtor/investor from buying a property out from under you if they show it and then think...hmmm, that would be a great addition to my portfolio.?