The offsides rule in football is different. It's not the same rule. The soccer offsides is the equivalent of penalizing a WR for getting deeper than a DB (you have to keep defenders between yourself and the goal). The football offsides only relates to what happens before the play is live. I'd assume soccer has a similar rule for mid-field kickoffs (or whatever they're called). Of course you can't start with your offensive players past the ball. I get that.
I just don't understand why the offense should yield that type of advantage to the defense that they (the offensive player) has to be responsible for what should be the job of the defender by rule. What would happen without it?