Probably because they are FSBO (For sale by owner). I have had many clients frustrated as they will be looking online and then when we go out they will see for sale signs that we never saw online. Or there will be short sales or the property is already Active Contingent, or even sold. Sites like Trulia.com and Zillow.com are great for free sites but if interested in looking at what is available, and being able to FILTER the inventory, there is no better way than to work with a local agent or Realtor. Best of luck.