too good to be true? What am i missing here?

[Mason B.]

Hello from Chicago,
   New to real-estate and potentially interested in starting with multi-unit properties here in Chicago. I been doing some light searching around and found some pretty impressive numbers with relatively low capital needed to achieve. 

Example: one of many with similar numbers. 
https://www.loopnet.com/Listing/5902-W-Ohio-St-Chicago-IL/13763945/

This 9 unit building is $425k and generates $49k net income. With $125,000 down the mortgage is $1500 month. $1,500 x 12(months) = $18,000 annually. 

$49,000(net income) - 18,000(mortgage expenses) = 31,000 profit.

that's a 30% return... (excluding appreciation) What am i missing? There seems to be many buildings producing these numbers, and why doesn't everybody just buy them all...?

*** I have free time and want to start a real-estate business. Goal is scale-ability and to re-invest cash flow into more properties, compounding the processes.