Because business doesnt have fix income.. in a month you can gain let say $5000 then the next month $8000 and improving.. And getting second job is not a good idea because you will have to spend your lifetime working... Listen to the micosoft's quote "work smart not harder" -
second is to answer the question.. Why do people work? - and the answer is to gain money and NEVER "to fill my free time or to have fun etc."- It's definitely to MAKE MONEY.
People now can make money online instead of getting a second job
http://www.nixquadre.co.nr because getting a second job is like killing off your days with your family (suicide)
however - to conclude one should really have a "primary job" and then he/she can do online work / business to make extra money.