1) they are presumably expanding to new markets and increasing their presence in current markets. That investment costs money and sometimes (in the beginning) surpasses profits. 2) Companies like Airbnb and Uber always have to deal with lawsuits and regulatory issues due to their nature. They have to spend money on legal issues in any market with different laws and real estate (in Airbnb’s case) regulations. 3) There are plenty of situations where they can lose money when there are customer or host issues, refunds, claims made, etc.
These are just a few. Are you that naive to not be able to think of ways a business can lose money?
While they are currently profitable, they still “lose money” in different ways, just like any other company can. Nothing is pure profit.