Because he is in debt to his eyeballs on those rental properties. In 30 years when they are paid off he will be rich, but right now he probably can't make ends meet without that income.
In the long run you make a lot of money, but it is a long time before profits are enough to live off of. For the first years the money goes to paying the bank, insurance, taxes, and if you are lucky there is enough left to cover basic repairs.