Because in cricket they're looking at where the ball may have gone - they're using technology to predict the path of the ball had it not hit the pad. It's not a matter of fact, it's a prediction and as the technology cannot predict with 100% certainty they allow a buffer to compensate for this. The exact same system is used in tennis but in tennis it's used to determine matter of fact decisions, not opinions or predictions and in tennis there is no buffer.In cricket there is a similar bias built in. It doesn't have to be like tennis.
Also that is only if the technology is accurate to 1mm in the first place. For balls and lines it is easy. For people it is a lot harder.
Whether a player is in an offside position is a matter of fact and with all the billions of pounds in football, if they can't develop a system that is accurate down to 1mm then they may as well pack VAR in all together. Hawk-eye have got a system that can do it down to 1mm with serves travelling at 140mph+.