Nick, a professional sports bettor will consistently beat the odds set by the sportsbook. As you say, a sportsbook makes money because, whichever team wins, they will – on average – take in more than they lose. This is not true for professionals, who will on average win more than they lose.
If every sports bettor was a skilled professional, all sportsbooks would be out of business.
That is why the vast majority of sportsbooks will close the accounts of professional gamblers, just as casinos will refuse to serve card counters, or (online) bonus abusers.
In any gambling venture where the player is competing against the house, it is profitable for the house only when they have an edge. A professional only exists because he can beat that edge, therefore professionals are not welcome at any such establishment.
Poker is different beause players compete against each other, and therefore the poker site wants skilled players – skilled players make them more money.
So while I am not in an way disputing that succesful sports betting takes skill, this is not an argument which the sports sites would ever use to protect themselves as the poker sites have done.