The first soccer referees were priests (hence the black referee uniforms from back in the day). It was seen as a moral issue to try for a goal that you did not earn.

The reality is, that if there was no offside, the attacking team would put a player or two back by the attacking goal. Then the defending team would have to compensate. That means you have one or more defensive players back by the goal the entire game. Breakaways would no longer exist. Your defense would have to spread to cover all of that area in the defensive half and open runs behind the backline would be a thing of the past. The game would get incredibly boring. (Even more boring for some of you).