Bookies do tend to move the spread in order to balance the betting from the public. This minimizes their risk. If they get exactly the same number of bets on each side of the play, they are guaranteed profit.
However, if the public is seriously wrong, the bookie may decide to keep the more accurate spread (in his opinion), or to move it but not all the way. But this makes the bookie a gambler - essentially, he’s betting on the game himself now. So the bookie generally has to feel that the public is not just wrong, but so wrong that it’s worth throwing his own money into the hat.
But there’s another problem with this. If the bookie moves his line differently than other bookies do, he can get ‘middled’. A middle is where a gambler finds two bookies with spreads so different that the gambler can bet one way with one bookie, and bet the opposing way with the second, and lower or eliminate his risk. This will allow him to bet large sums of money. Bookies don’t like being middled.
You CAN make a profit betting sports, and I know lots of people that do it for a living. There are some professional syndicates that have made millions doing it. But you have to have the gift of being able to spot lines that are wrong, which means you need to be able to accurately handicap yourself. That is not easy. The guys I know who do it seriously basically spend most of their time studying teams, statistics, matchups, historical results, etc.
Generally, pros beat the house by knowing something the house doesn’t. Remember, the pro can study just one game, while the bookies have to set lines for hundreds of games that may be going on that day in many different sports. For example, you may know that the weather forecast calls for rain, and that this disadvantages one team more than another, based on their style of play.
The internet makes it all tougher, because bookies communicate with each other now. One common way to beat the house in the old days was to exploit the ‘home team support’ angle. Basically, the population of a city where the home team is playing will tend to bet with the home team, which moves the line away from where it ‘should’ be. Gamblers used to exploit that by betting against the home team with the local bookie, or betting with them with a bookie in the opposing team’s city, getting better odds than the locals are. Or there might be a good middle opportunity.
Today, because bookies communicate, the local bookie will tend to leave his line unbalanced in order to keep his spread the same as other bookies, which means the local lines are now more accurate.
Finally, the true professional sports bettor does not bet every game, or even anywhere close to it. It’s a tedious business, because you have to spend a lot of time analyzing games, only to find that the bookie’s line is pretty much correct. So all that work goes down the drain, and you start another. Some pros will only bet one or two games for every 20 or 30 they analyze.
Btw, you can also beat blackjack, contrary to what DRS said.