The odds are designed in a way to attracts the most action on both sides of the bet. The ultimate goal is to have a perfect equal volume on both sides, however, that is never the case. Additionally, books factor in a 5-10% margin, that ensures their profits. Basically saying, you can look at the oddsmakers as risk management professionals and not just sports predictors.
All of the above has been laid into the foundation since the very first day of bookmaking craft. The only difference was that back in a day, they did not have any computer or advanced software. Everything was done manually by hand or generated through a thoughtful brainstorming process.