There can be a lot of reasons for this. Here are a few from my personal experience in investing.
1) The expenses and depreciation offset the income to a point that your company really isn't making any net income. In the GTA cap rates suck, but appreciation has made up for it, this means you aren't paying taxes on income at the end of the year. (take recapture into account)
2) Part of a tax saving strategy in passing the portfolio down to your children.
3) Sale of a startup business and cash sitting in a corporation, withdrawing it would have tax implications so it's invested through the corporation and this kind of looped back into point # 1.
4) You pay half of your money to the government anyway on personal taxes.
It all depends on your exit strategy, your personal financial situation and your future plans with the property.
For larger groups with investors or for larger apartment buildings there are a whole bunch of different reasons, but I assume those won't apply to you.
Hope that provides a bit of insight.
(I'm not an accountant haha)