Because they had a service fee. When you wager for example $100 and if you will win you have a profit of $90. So, the $10 is their revenue.
But you will lose the entire $100 will consider of course as a loss. I would say 75% skills and 25% based on luck, I guess that's sports betting work and it should be the long term is based on skill, it requires more skill than luck like live poker. While on other the hand, in the short term, luck will be a much larger factor.

If you are basing on pure luck will bring you nothing but a loss and disappointment. Digging more analysis and research the team and their previous records will increase your chances of winning. Together with hard work and great discipline are required to profit in sports betting.