That cheap fare you just booked will look different when you go to pay for it. In addition to taxes, your final price includes government-imposed fees for security, immigration, customs and agriculture inspections. It may also include fees for services and baggage. If you have to change your flight, you'll get hit with more fees.
Because airlines aren't required to include these fees in the ticket price, you don't learn about them until it's time to pay. There's a bonus for the airline, too. By separating fees from ticket prices, airlines avoid paying taxes on billions of dollars in revenue.