Odds providers have their models on how likely each outcome is. They add a spread to this (for example if it's 50/50, they'll pay out 1.8x instead of 2x). The line can also be affected by other factors, like how likely it is one side is oversubscribed. They don't make money on all markets, just most

But if someone has a model that is so significantly better than theirs that it beats the line _and_ spread, they will make money in the long run. Haralabos Voulgaris for example is likely one of the most successful sports bettors. Very interesting guy imo