Because profit requires you to pay taxes. It's not uncommon at all for growing companies to invest and otherwise use up any profit they generate so the balance sheet stays negative while the company keeps growing in value.
Also allows you to cry how those dirty 3rd party app developers are stealing all your profit boo hoo.
When you look at the revenue growth of reddit, it's not hard to see that if they were able to function at all when they had a revenue 1/10th of what it's now, they could turn profit if they wanted to.