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.
That's wrong, most gambling sites offers a lot of odds, there are odds like 2.00 and over, so if you will keep choosing that odds, you will always get 100% or more from the amount you bet. That's what we called juice or commission in sports betting, but like I've stated, you can always do away with that.
I would like to demonstrate an example.
check this odds,
you can bet on brooklyn nets for 3.5 to get the odds of 2.05 and if you go on the other side, you can bet on the Lakers at -5.5 and you will get an odds of 2.05.