Why is it necessary to think defence first?

Just about every new tactic since the mid 90s has been a defensive one. This is held in the belief/viewpoint that a good defence will win matches. I would like to know why that is? Is there any science behind it? I do believe it is to do with the fact that finals lift a cog in intensity, so it means that all out attack won't work in a final. Why can't there be a mix of both - tough in close footy and once you get the ball out, it's straight for the goal? Rather than pre-meditated two or three extra men in the backlines?
Has anyone on here coached a side (any side) and thought about what the best way to win a match is and why?
Has anyone on here coached a side (any side) and thought about what the best way to win a match is and why?