First, 50,000 points is not the same as 50,000 dollars. Most airline companies allow you to buy (or top up) your points for money, and you'll end up paying around $10-20 for 1,000 points, therefore 50,000 points equates to $500-$1000 in money.
Next, to answer your actual question, you need to understand how airline tickets are priced. The cost of the ticket is composed of several items:
- actual cost of the ticket that the airline charges you
- government imposed taxes (e.g. X dollars for each short leg and Y dollars for each long leg)
- airport imposed fees (e.g. Z dollars for each outgoing flight)
- airline imposed surcharges (e.g. fuel surcharge, etc.)
Overall, the actual price of the flight is often a quarter of the total amount you have to pay for your ticket.
Now, when you are using your loyalty points to pay for the ticket, these points can only be applied toward the actual cost of the ticket (i.e. the first bullet point above). You still have to pay money for everything else.
As an example, a couple of years ago, I used loyalty points to pay for a business class ticket from London to Sydney on Virgin Atlantic. I paid 200,000 points plus about over $1000 in money for taxes and fees. On the same trip, I used loyalty points for an internal flight within Australia - I paid 30,000 points plus another about $50 for taxes, fees, etc.
Hopefully, this gives you an idea of how reward tickets (that's what they are called) work.